pgsql: Marginal improvement in sublink planning: allow unknownEqFalse

Log Message:
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Marginal improvement in sublink planning: allow unknownEqFalse optimization
to be used for SubLinks that are underneath a top-level OR clause. Just as at
the very top level of WHERE, it's not necessary to be accurate about whether
the sublink returns FALSE or NULL, because either result has the same impact
on whether the WHERE will succeed.

Modified Files:
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pgsql/src/backend/optimizer/plan:
subselect.c (r1.136 -> r1.137)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/subselect.c?r1=1.136&r2=1.137)