when it should use index

Sometimes the LEFT/RIGHT JOIN with an empty table caused an
unnecessary filesort.

Sample query, where t1.i1 is indexed and t3 is empty:

  SELECT t1.*, t2.* FROM t1 JOIN t2 ON t1.i1 = t2.i2
                       LEFT JOIN t3 ON t2.i2 = t3.i3
    ORDER BY t1.i1 LIMIT 5;

The server erroneously used an item of empty outer-joined
table as a common constant of a Item_equal (multi-equivalence
expression).
By the fix for the bug 16590 the constant status of such
an item has been propagated to st_table::const_key_parts
map bits related to other Item_equal argument-related
key parts (those are obviously not constant in our case).
As far as test_if_skip_sort_order function skips constant
prefixes of testing keys, this caused an ignorance of
available indices, since some prefixes were marked as
constant by mistake.