Customer's test case for bug#12695 Item_func_isnull::update_used_tables() did 
  not update const_item_cache.


mysql-test/t/insert_select.test:
  Customer's test case for bug#12695 Item_func_isnull::update_used_tables() did 
  not update const_item_cache.
mysql-test/r/insert_select.result:
  Customer's test case for bug#12695 Item_func_isnull::update_used_tables() did 
  not update const_item_cache.