A clean-up for c8141f53

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.

sql/item.h

@@ -3575,6 +3575,22 @@ class Item_ref :public Item_ident
   Item *get_tmp_table_item(THD *thd);
   table_map used_tables() const;		
   void update_used_tables(); 
+  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
+                          bool link_item_fields)
+  {
+    /*
+      Item_ref cannot refer to Item_field when build_equal_items() is called,
+      because all "WHERE/HAVING field" are already replaced to
+      "WHERE/HAVING field<>0" by this time. See normalize_cond().
+      TODO: make sure with Igor and Sanja, perhaps the below expression
+      can be simplified just to a Item::build_equal_items() call.
+    */
+    return real_type() == FIELD_ITEM ?
+           Item_ident_or_func_or_sum::build_equal_items(thd, inherited,
+                                                        link_item_fields) :
+           Item::build_equal_items(thd, inherited,
+                                   link_item_fields);
+  }
   bool const_item() const 
   {
     return (*ref)->const_item();

sql/item_sum.h

@@ -444,6 +444,17 @@ class Item_sum :public Item_func_or_sum
   */
   table_map used_tables() const { return used_tables_cache; }
   void update_used_tables ();
+  COND *build_equal_items(THD *thd, COND_EQUAL *inherited,
+                          bool link_item_fields)
+  {
+    /*
+      Item_sum (and derivants) of the original WHERE/HAVING clauses
+      should already be replaced to Item_aggregate_ref by the time when
+      build_equal_items() is called. See Item::split_sum_func2().
+    */
+    DBUG_ASSERT(0);
+    return Item::build_equal_items(thd, inherited, link_item_fields);
+  }
   bool is_null() { return null_value; }
   void make_const () 
   {