pgsql: Better handle pseudotypes as partition keys

Better handle pseudotypes as partition keys

We fail to handle polymorphic types properly when they are used as
partition keys: we were unnecessarily adding a RelabelType node on top,
which confuses code examining the nodes. In particular, this makes
predtest.c-based partition pruning not to work, and ruleutils.c to emit
expressions that are uglier than needed. Fix it by not adding RelabelType
when not needed.

In master/11 the new pruning code is separate so it doesn't suffer from
this problem, since we already fixed it (in essentially the same way) in
e5dcbb88a15d, which also added a few tests; back-patch those tests to
pg10 also. But since UPDATE/DELETE still uses predtest.c in pg11, this
change improves partitioning for those cases too. Add tests for this.
The ruleutils.c behavior change is relevant in pg11/master too.

Co-authored-by: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
Co-authored-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>
Reviewed-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Discussion: https://postgr.es/m/54745d13-7ed4-54ac-97d8-ea1eec95ae25@lab.ntt.co.jp

Branch
------
REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e7df94f317c979ad176adc0155a3e8af63e68ac9

Modified Files
--------------
src/backend/partitioning/partbounds.c | 50 +++++++++++----------------
src/test/regress/expected/create_table.out | 2 +-
src/test/regress/expected/partition_prune.out | 18 ++++++++++
src/test/regress/sql/partition_prune.sql | 2 ++
4 files changed, 41 insertions(+), 31 deletions(-)