MDEV-8754 Wrong result for SELECT..WHERE year_field=2020 AND NULLIF(year_field,2010)='2020'
Problems:
1. Item_func_nullif stored a copy of args[0] in a private member m_args0_copy,
   which was invisible for the inherited Item_func menthods, like
   update_used_tables(). As a result, after equal field propagation
   things like Item_func_nullif::const_item() could return wrong result
   and a non-constant NULLIF() was erroneously treated as a constant
   at optimize_cond() time.
   Solution: removing m_args0_copy and storing the return value item
   in args[2] instead.
2. Equal field propagation did not work well for Item_fun_nullif.
   Solution: using ANY_SUBST for args[0] and args[1], as they are in
   comparison, and IDENTITY_SUBST for args[2], as it's not in comparison.