MDEV-7950 Item_func::type() takes 0.26% in OLTP RO (Step#2)

- Item_ref was doing unnecessary extra job after the "MDEV-7950 Step#2" patch.
  Fallback to Item::build_equal_items() if real_type() is not FIELD_ITEM.
  Note, Item_ref::build_equal_items() will most likely be further simplified.
  Waiting for Sanja and Igor to check a possibly dead code.

- Safety: Adding Item_sum::build_equal_items() with ASSERT, as Item_sum
  should never appear in build_equal_items() context.