pgsql: Teach predtest.c that "foo" implies "foo IS NOT NULL".

Teach predtest.c that "foo" implies "foo IS NOT NULL".

Per complaint from Peter Holzer. It's useful to cover this special case,
since for a boolean variable "foo", earlier parts of the planner will have
reduced variants like "foo = true" to just "foo", and thus we may fail
to recognize the applicability of a partial index with predicate
"foo IS NOT NULL".

Back-patch to 9.5, but not further; given the lack of previous complaints
this doesn't seem like behavior to change in stable branches.

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/8dccf030e884ea8c723275a070acf8a8ed1eebe1

Modified Files
--------------
src/backend/optimizer/util/predtest.c | 4 ++++
1 file changed, 4 insertions(+)

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