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

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