Investigate renewed in 10.3 CASE implementation.

There is an accessor sql/item_cmpfunc.h:Item_func_case::get_first_expr_num() in forked server code that is used to determine whether the CASE is simple or searched. This accessor should be removed if it is possible.

class Item_func_case :public Item_func_hybrid_field_type

...

get_first_expr_num() { return first_expr_num; }