Problem:
--------
This problem occurs under the following conditions:
1) Table has a unique index.
2) Query checks whether a loose index scan is applicable to
this query. And mistakenly chooses it despite unique
indexes having no index extensions.

This is can give wrong results. A similar problem also
exists in Bug #26532061.

Solution:
---------
Index extensions are not applicable to UNIQUE indexes for
loose index scans.

So Field::is_part_of_actual_key should also consider the
HA_NOSAME flag.