Step 2c:

After discussion with Igor, it appeared that Item_field and Item_ref
could not appear in this context in the old function build_equal_item_for_cond:

  else if (cond->type() == Item::FUNC_ITEM ||
           cond->real_item()->type() == Item::FIELD_ITEM)

The part of the condition checking for Item_field::FIELD_ITEM was a dead code.
- Moving implementation of Item_ident_or_func_or_sum::build_equal_items()
to Item_func::build_equal_items()
- Restoring deriving of Item_ident and Item_sum_or_func from Item_result_field.
  Removing Item_ident_or_func_or_sum.