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
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REL_14_STABLE

Details
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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(-)