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

Step1 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 single_value_in_to_exists_transformer() into two methods:
- create_single_value_in_to_exists_cond(), and
- inject_single_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

@@ -425,8 +425,18 @@ class Item_in_subselect :public Item_exists_subselect
   trans_res select_in_like_transformer(JOIN *join, Comp_creator *func);
   trans_res single_value_transformer(JOIN *join, Comp_creator *func);
   trans_res row_value_transformer(JOIN * join);
+
   trans_res single_value_in_to_exists_transformer(JOIN * join,
                                                   Comp_creator *func);
+  trans_res create_single_value_in_to_exists_cond(JOIN * join,
+                                                  Comp_creator *func,
+                                                  Item **where_term,
+                                                  Item **having_term);
+  trans_res inject_single_value_in_to_exists_cond(JOIN * join,
+                                                  Comp_creator *func,
+                                                  Item *where_term,
+                                                  Item *having_term);
+
   trans_res row_value_in_to_exists_transformer(JOIN * join);
   virtual bool exec();
   longlong val_int();