MWL#89: Cost-based choice between Materialization and IN->EXISTS transformation

  
Step2 in the separation of the creation of IN->EXISTS equi-join conditions from
their injection. The goal of this separation is to make it possible that the
IN->EXISTS conditions can be used for cost estimation without actually modifying
the subquery.
  
This patch separates row_value_in_to_exists_transformer() into two methods:
- create_row_value_in_to_exists_cond(), and
- inject_row_value_in_to_exists_cond()
The patch performs minimal refactoring of the code so that it is easier to solve
problems resulting from the separation. There is a lot to be simplified in this
code, but this will be done separately.

sql/item_subselect.h

@@ -438,6 +438,13 @@ public:
                                                   Item *having_term);
 
   trans_res row_value_in_to_exists_transformer(JOIN * join);
+  trans_res create_row_value_in_to_exists_cond(JOIN * join,
+                                               Item **where_term,
+                                               Item **having_term);
+  trans_res inject_row_value_in_to_exists_cond(JOIN * join,
+                                               Item *where_term,
+                                               Item *having_term);
+
   virtual bool exec();
   longlong val_int();
   double val_real();