pgsql: Add ssup_datum_uint32_cmp for comparing OIDs

Add ssup_datum_uint32_cmp for comparing OIDs

Commit 51cd5d6f0 used ssup_datum_unsigned_cmp for the OID comparator,
which compares entire Datums. That gave wrong results, since the upper
half of a Datum containing a 32-bit type is not reliably zero-extended:
values fetched from tuples are sign-extended, while values returned
by e.g. oidin() are zero-extended. An OID with the high bit set could
therefore compare as larger or smaller depending on its source.

Fix by adding a sort support comparator that only looks at the low 32
bits. Also teach radix sort to normalize Datums the same way. This keeps
OID eligible for radix sort. Add a regression test that sorts OIDs above
2^31 from both kinds of source, taken from Peter Geoghegan's report.

Reported-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reported-by: Peter Geoghegan <pg(at)bowt(dot)ie>
Suggested-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Discussion: https://postgr.es/m/CAN4CZFM2afRzSLJJiWKnRjqPwL7Scm2Wd1xXu140diJGuJtzGg@mail.gmail.com
Discussion: https://postgr.es/m/CAH2-WznvXyKRah=MQ3ONGHiN8ahJcqpmtyikxCOO96A4usDPpg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/94d32ce1a63c67bce7b811207095ee6d41d4caed

Modified Files
--------------
src/backend/access/nbtree/nbtcompare.c | 2 +-
src/backend/utils/sort/tuplesort.c | 38 ++++++++++++++++++++++++----------
src/include/utils/sortsupport.h | 1 +
src/test/regress/expected/oid.out | 16 ++++++++++++++
src/test/regress/sql/oid.sql | 4 ++++
5 files changed, 49 insertions(+), 12 deletions(-)