MDEV-19134: EXISTS() slower if ORDER BY is defined

Step 1: Removal of ORDER BY [LIMIT] from the subquery should be done
earlier and for broader class of subqueries.

The rewrite was done in Item_in_subselect::select_in_like_transformer(),
but this had problems:
- It didn't cover EXISTS subqueries
- It covered IN-subqueries, but was done after the semi-join transformation
  was considered inapplicable, because ORDER BY was present.

Remaining issue:
- EXISTS->IN transformation happens before
  check_and_do_in_subquery_rewrites() is called, so it is still prevented
  by the present ORDER BY.

mysql-test/main/union.result

@@ -2049,7 +2049,7 @@ insert t1 values (1),(2),(3),(1);
 explain select 1 from dual where exists (select max(a) from t1 group by a union select a+2 from t1);
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
-2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	4	Using temporary; Using filesort
+2	SUBQUERY	t1	ALL	NULL	NULL	NULL	NULL	4	Using temporary
 3	UNION	t1	ALL	NULL	NULL	NULL	NULL	4	
 NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
 drop table t1;