pgsql: Fix incorrect hash equality operator bug in Memoize

Fix incorrect hash equality operator bug in Memoize

In v14, because we don't have a field in RestrictInfo to cache both the
left and right type's hash equality operator, we just restrict the scope
of Memoize to only when the left and right types of a RestrictInfo are the
same.

In master we add another field to RestrictInfo and cache both hash
equality operators.

Reported-by: Jaime Casanova
Author: David Rowley
Discussion: https://postgr.es/m/20210929185544.GB24346%40ahch-to
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1f194ed6c26e67ce92437729fc46193cc6e913d2

Modified Files
--------------
src/backend/optimizer/plan/initsplan.c | 19 +++++++++++++++----
1 file changed, 15 insertions(+), 4 deletions(-)