(for example, such objects can be returned from get_mm_parts() for "string_field = int_val).
Make find_used_partitions_imerge() to handle such SEL_TREE objects.


mysql-test/r/partition_pruning.result:
  BUG#19055: testcase
mysql-test/t/partition_pruning.test:
  BUG#19055: testcase