pgsql: Teach hash_ok_operator() that record_eq is only sometimes hashab

Teach hash_ok_operator() that record_eq is only sometimes hashable.

The need for this was foreseen long ago, but when record_eq
actually became hashable (in commit 01e658fa7), we missed updating
this spot.

Per bug #17363 from Elvis Pranskevichus.  Back-patch to v14 where
the faulty commit came in.

Discussion: https://postgr.es/m/17363-f6d42fd0d726be02@postgresql.org

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/17da9d4c28297fd699cbbda969a9f64c4c09c665

Modified Files
--------------
src/backend/optimizer/plan/subselect.c  |  6 +++---
src/test/regress/expected/subselect.out | 23 +++++++++++++++++++++++
src/test/regress/sql/subselect.sql      | 10 ++++++++++
3 files changed, 36 insertions(+), 3 deletions(-)