MDEV-37438: UBSAN add_ext_keyuse_for_splitting downcast Item_func_equal -> Item_func_eq

add_ext_keyuse_for_splitting assumed the added_key_field->cond was
of type Item_func_eq. In 11.8 there are cases where this is actually
of type Item_func_equal in some cases which is not a class descendant
of Item_func_eq.

The only use of this class is accessing its arguments() method
which is defined all the way up the hierarchy of both Item_func_equal
and Item_func_eq in the class Item_args.

To resolve future mis-castings of this method we assume the topmost
class of Item_args.

Author: grooverdan

sql/opt_split.cc

@@ -649,7 +649,7 @@ add_ext_keyuse_for_splitting(Dynamic_array<KEYUSE_EXT> *ext_keyuses,
   possible_keys.intersect(field->table->keys_usable_for_splitting);
   tab->keys.merge(possible_keys);
 
-  Item_func_eq *eq_item= (Item_func_eq *) (added_key_field->cond);
+  Item_args *eq_item= (Item_args *) (added_key_field->cond);
   keyuse_ext.table= field->table;
   keyuse_ext.val= eq_item->arguments()[1];
   keyuse_ext.key= key;