pgsql: Teach predicate_refuted_by() how to do proofs involving

Log Message:
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Teach predicate_refuted_by() how to do proofs involving NOT-clauses.
This doesn't matter too much for ordinary NOTs, since prepqual.c does
its best to get rid of those, but it helps with IS NOT TRUE clauses
which the rule rewriter likes to insert. Per example from Martin Lesser.

Modified Files:
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pgsql/src/backend/optimizer/util:
predtest.c (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c.diff?r1=1.7&r2=1.8)