Fix incorrect zero extension of Datum in JIT tuple deform code

Fix incorrect zero extension of Datum in JIT tuple deform code

When JIT deformed tuples (controlled via the jit_tuple_deforming GUC),
types narrower than sizeof(Datum) would be zero-extended up to Datum
width.  This wasn't the same as what fetch_att() does in the standard
tuple deforming code.  Logically the values are the same when fetching
via the DatumGet*() marcos, but negative numbers are not the same in
binary form.

In the report, the problem was manifesting itself with:

ERROR: could not find memoization table entry

in a query which had a "Cache Mode: binary" Memoize node. However, it's
currently unclear what else is affected.  Anything that uses
datum_image_eq() or datum_image_hash() on a Datum from a tuple deformed by
JIT could be affected, but it may not be limited to that.

The fix for this is simple: use signed extension instead of zero
extension.

Many thanks to Emmanuel Touzery for reporting this issue and providing
steps and backup which allowed the problem to easily be recreated.

Reported-by: Emmanuel Touzery <emmanuel.touzery@plandela.si>
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/DB8P194MB08532256D5BAF894F241C06393F3A@DB8P194MB0853.EURP194.PROD.OUTLOOK.COM
Backpatch-through: 13