pgsql: 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(dot)touzery(at)plandela(dot)si>
Author: David Rowley <dgrowleyml(at)gmail(dot)com>
Discussion: https://postgr.es/m/DB8P194MB08532256D5BAF894F241C06393F3A@DB8P194MB0853.EURP194.PROD.OUTLOOK.COM
Backpatch-through: 13

Branch
------
REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/4afab175bc13a08277ad40400a11db4e16905547

Modified Files
--------------
src/backend/jit/llvm/llvmjit_deform.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)