* When executing EXPLAIN, do the same as for the query: convert join type to JT_CONST if keyuse array covers all key parts and all of them are constants.
 * In remove_const, don't remove conditions that depend on some-const-table and current-table.


mysql-test/r/join_outer.result:
  Testcase for bug#5088
mysql-test/t/join_outer.test:
  Testcase for bug#5088