Analysis:
During optimization of the subquery, in the call chain:

update_ref_and_keys -> add_key_fields ->
merge_key_fields -> Item_direct_ref::is_null -> Item_cache::is_null

The call to Item_cache::is_null() returns TRUE, which is wrong.
This results in Item_null replacing the field 'f3' in the KEY_FIELD,
then this Item_null is used for index access, producing a wrong result.

The reason why Item_cache::is_null returns wrong result is that
this Item_cache object is a cache of the left operand of IN, and was
updated in Item_in_optimizer::val_int. In MWL#89 the latter method is
called during the execution phase, which is after we optimize the subquery.
Therefore during the optization phase the left operand cache of IN was
not updated.

Solution:
Update the left operand cache during optimization if it is a constant.

This bug fix also discoveres and fixes a wrong IF statement in
convert_constant_item().