pgsql: Another go at making pred_test() handle all reasonable

Log Message:
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Another go at making pred_test() handle all reasonable combinations
of AND and OR clauses.  The key point here is that an OR on the
predicate side has to be treated gingerly: we may be able to prove
that the OR is implied even when no one of its components is implied.
For example (x OR y) implies (x OR y OR z) even though no one of x,
y, or z can be individually proven.  This code handles both the
example shown recently by Sergey Koshcheyev and the one shown last
October by Dawid Kuroczko.

Modified Files:
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    pgsql/src/backend/optimizer/path:
        indxpath.c (r1.168 -> r1.169)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.168&r2=1.169)