VALUES() was considered a constant. This caused replacing 
 (or pre-calculating) it using uninitialized values before the actual
 execution takes place.
 Mark it as a non-constant (still not dependent of tables) to prevent
 the pre-calculation.


mysql-test/r/insert_update.result:
  Bug#21555: incorrect behavior with INSERT ... ON DUPL KEY UPDATE and VALUES
   - test case.
   - EXPLAIN output changed due to VALUES() not being considered a constant 
     anymore
mysql-test/t/insert_update.test:
  Bug#21555: incorrect behavior with INSERT ... ON DUPL KEY UPDATE and VALUES
   - test case.
sql/item.h:
  Bug#21555: incorrect behavior with INSERT ... ON DUPL KEY UPDATE and VALUES
   - mark Item_insert_value as non-constant to prevent early calculation.