pgsql: Improve predicate_refuted_by_simple_clause() to handle IS NULL

Log Message:
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Improve predicate_refuted_by_simple_clause() to handle IS NULL and IS NOT NULL
more completely. The motivation for having it understand IS NULL at all was
to allow use of "foo IS NULL" as one of the subsets of a partitioning on
"foo", but as reported by Aleksander Kmetec, it wasn't really getting the job
done. Backpatch to 8.2 since this is arguably a performance bug.

Tags:
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REL8_2_STABLE

Modified Files:
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pgsql/src/backend/optimizer/util:
predtest.c (r1.10 -> r1.10.2.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c.diff?r1=1.10&r2=1.10.2.1)