UPPER(key_col) IN (constant subquery) does not use the index

Item_func_in::varchar_upper_cmp_transformer() rewrites
UPPER(key_col) IN (const, ...) into key_col IN (const, ...).

It needs a copy of the IN predicate to put the new first argument into, it used deep_copy_with_checks(). A deep copy copies every node of the tree and refuses as a whole if any node cannot be copied, so an IN list holding an uncorrelated scalar subquery, which is constant but has no copy, silently lost the rewrite: UPPER(a) IN ((SELECT MAX(c) FROM t2),'A-2') read the whole table instead of using a range on the index on a. After copying, the rewrite walked the copy with cleanup_excluding_const_fields_processor and re-fixed it, which would also have cleaned up a subquery had one been copied.