Improve predtest.c's handling of cases with NULL-constant inputs.

Currently, if operator_predicate_proof() is given an operator clause like
"something op NULL", it just throws up its hands and reports it can't prove
anything.  But we can often do better than that, if the operator is strict,
because then we know that the clause returns NULL overall.  Depending on
whether we're trying to prove or refute something, and whether we need
weak or strong semantics for NULL, this may be enough to prove the
implication, especially when we rely on the standard rule that "false
implies anything".  In particular, this lets us do something useful with
questions like "does X IN (1,3,5,NULL) imply X <= 5?"  The null entry
in the IN list can effectively be ignored for this purpose, but the
proof rules were not previously smart enough to deduce that.

This patch is by me, but it owes something to previous work by
Amit Langote to try to solve problems of the form mentioned.
Thanks also to Emre Hasegeli and Ashutosh Bapat for review.

Discussion: https://postgr.es/m/3bad48fc-f257-c445-feeb-8a2b2fb622ba@lab.ntt.co.jp

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