Backport of WL#5953 from MySQL 5.6

The patch differs from the original MySQL patch as follows:
- All test case differences have been reviewed one by one, and
  care has been taken to restore the original plan so that each
  test case executes the code path it was designed for.
- A bug was found and fixed in MariaDB 5.3 in
  Item_allany_subselect::cleanup().
- ORDER BY is not removed because we are unsure of all effects,
  and it would prevent enabling ORDER BY ... LIMIT subqueries.
- ref_pointer_array.m_size is not adjusted because we don't do
  array bounds checking, and because it looks risky.

Original comment by Jorgen Loland:
-------------------------------------------------------------
WL#5953 - Optimize away useless subquery clauses
      
For IN/ALL/ANY/SOME/EXISTS subqueries, the following clauses are 
meaningless:
      
* ORDER BY (since we don't support LIMIT in these subqueries)
* DISTINCT
* GROUP BY if there is no HAVING clause and no aggregate 
  functions
      
This WL detects and optimizes away these useless parts of the
query during JOIN::prepare()

mysql-test/suite/innodb_plugin/r/innodb_mysql.result

@@ -1425,9 +1425,9 @@ DROP TABLE t1;
 create table t1 (a bit(1) not null,b int) engine=myisam;
 create table t2 (c int) engine=innodb;
 set @save_optimizer_switch=@@optimizer_switch;
-set @@optimizer_switch='partial_match_rowid_merge=off,partial_match_table_scan=off';
+set @@optimizer_switch='in_to_exists=on,materialization=off';
 explain
-select b from t1 where a not in (select b from t1,t2 group by a) group by a;
+select b from t1 where a not in (select max(b) from t1,t2 group by a) group by a;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
 2	DEPENDENT SUBQUERY	t1	system	NULL	NULL	NULL	NULL	0	const row not found