This patch mainly adds sorting of all indexes for partial matching
according to their NULL selectivity. The patch also fixes a related bug
in subselect_rowid_merge_engine::test_null_row() where the wrong matched
indexes were skipped.

In addition the patch:
- adds few ::print() methods,
- renames few variables that had similar names but different purpose.

for table elimination in debug builds.
(part 2)

for table elimination in debug builds.

In Item_ref::fix_fields() do invoke mark_as_dependent() for outside 
references in all cases (see email for more details)

sql/item.cc:
  In Item_ref::fix_fields() do invoke mark_as_dependent() for outside references in all cases.
sql/item.h:
  Change Field_enumerator to enumerate Item_field-s not Field-s.
sql/item_subselect.cc:
  Change Field_enumerator to enumerate Item_field-s not Field-s.
sql/opt_table_elimination.cc:
  Change Field_enumerator to enumerate Item_field-s not Field-s.

  Don't go into branch that calls upper_refs.empty() more than once per
  PREPARE or EXECUTE
* Avoid crashing when processing references to outside from subquery's HAVING
  (will explain in more details in email)

sql/item.h:
  * Avoid crashing when processing references to outside from subquery's HAVING
    (will explain in more details in email)
sql/item_subselect.cc:
  * Better self-recursion protection in Item_subselect::fix_fields. 
    Don't go into branch that calls upper_refs.empty() more than once per
    PREPARE or EXECUTE

prepared statements: re-collect list of upper refs on every PS re-execution.

This patch implements correct NULL semantics for materialized subquery execution.
The implementation has the following properties and main limitations:
- It passes all query result tests, but fails a number of EXPLAIN tests because of
  changed plans.
- The EXPLAIN output for partial matching is not decided yet.
- It works only when all necessary indexes fit into main memory. Notice that these
  are not the general B-tree/Hash indexes, but instead much more compact ones,
  therefore this limitation may not be a problem in many practical cases.
- It doesn't contain specialized tests.
- In several places the implementation uses methods that are modified copies of
  other similar methods. These cases need to be refactored to avoid code duplication.
- Add a test if the predicate is top-level just before deciding on partial matching.
  If it is top-level, use a more efficient exec method (index lookup).
- Add sorting of indexes according to their selectivity. The code is almost there.
- Needs more comments, and to sync existing ones with the implementation.

sql/item_cmpfunc.h:
  Expose the Arg_comparator of a comparison predicate. This makes it possible to
  directly get the comparison result {-1,0,1}, which is not possible through the
  val_XXX() methods which "fold" such results into a boolean.
sql/item_subselect.cc:
  The core of the implementation of MWL#68.
sql/item_subselect.h:
  The core of the implementation of MWL#68.
sql/opt_subselect.cc:
  Removed the limitation for materialized subquery execution that it is applicable only
  for top-level predicates.
sql/sql_class.cc:
  New class select_materialize_with_stats that collects data statistics about
  the data being inserted into the target table.
sql/sql_class.h:
  New class select_materialize_with_stats that collects data statistics about
  the data being inserted into the target table.
sql/sql_select.cc:
  - more complete initialization of the TABLE object of a temp table.
  - call setup_subquery_materialization at one more exit point.

- More test results updates (checked)

- Update test results
- More comments
- Add Item_in_optimizer::transform() which was lost in backport

- Factor out subquery code into sql/opt_subselect.{h,cc}
- Stop using the term "confluent" (was used due to misreading the dictionary)

- Variant #3 of the fix. It also
 = Unifies code with table elimination's 
 = is able to handle FROM-subquery pullout.

- Fix valgrind failure: do initialize Item::is_expensive_cache.

XOR conditions are not optimized, and Item_cond_xor therefore
acts like type Func_item even though it inherits from Item_cond.
A subtle difference between Item_func and Item_cond is that
you can get the children Items from the former by calling
arguments(), and from the latter by calling argument_list().
However, since Item_cond_xor inherits from Item_cond,
arguments() did not return any Items.

The fact that Item_cond_xor::arguments() did not return it's
children items lead to a problem for make_cond_for_index();
the method accepted that XOR items on unindexed columns were
pushed using ICP. ICP evaluation of non-indexed columns
does not (and should not) work.

The fix for this bug is to make Item_cond_xor return it's
children items when the arguments() method is used. This makes
Item_cond_xor behave more like Item_func and in turn allows
make_cond_for_index() to discover any conflicting children
Items.

This is a temporary fix and should be removed when Item_cond_xor
 is optimized.

process temporary table overflow correctly.

- Backport into Maria DB 5.3, part 1

- Enable semi-join handling in the join cache code

- Fix valgrind failure
- Test result fixes (not finished)

There are still test failures because of:
- Wrong query results in outer join + semi join
- EXPLAIN output differences

- Do call update_used_tables() for new conditions obtained when adding
  outer join's triggered conditions. Correct values of used_tables() are
  now needed for condition pushdown.
- Update test results

mysql-test/suite/pbxt/r/join_outer.result:
  DS-MRR backport:
  - Update test results
sql/sql_select.cc:
  DS-MRR backport: fix buildbot valgrind failures:
  - Do call update_used_tables() for new conditions obtained when adding
    outer join's triggered conditions. Correct values of used_tables() are
    now needed for condition pushdown.

- Add opt_range_mrr.cc file into source repo

mysql-test/r/myisam_mrr.result:
  Make testcase work for both debug and release
mysql-test/t/myisam_mrr.test:
  Make testcase work for both debug and release
sql/Makefile.am:
  - Add opt_range_mrr.cc file into source repo

- Make index condition pushdown be controlled by an @@optimizer_switch flag,
  not by @@engine_condition_pushdown
- Make MRR buffer size be controlled by @@mrr_buffer_size, not 
  by @@read_rnd_buffer_size
- Move parts of code to separate files
- Code cleanup
- Add --sorted_result to some SELECTs in tests.

WL#2771 "Block Nested Loop Join and Batched Key Access Join"

- Fix PBXT test results (PBXT doesn't support MRR or ICP, but we get result 
  diffs because we've also backported a fix that
  - prints out "Using where" when the table has part of WHERE that it has 
    got from LEFT JOIN's ON expression
  - Does a better job at removing equalities that are guaranteed to be true 
    by use of ref acccess.