pgsql: Fix oversight in indexscan plan creation.

Log Message:
-----------
Fix oversight in indexscan plan creation. I recently added code to use
predicate_implied_by() to detect redundant filter conditions, but forgot
that predicate_implied_by() assumes its first argument contains only
immutable functions. Add a check to guarantee that. Also, test to see
if filter conditions can be discarded because they are redundant with
the predicate of a partial index.

Modified Files:
--------------
pgsql/src/backend/optimizer/plan:
createplan.c (r1.198 -> r1.199)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/plan/createplan.c.diff?r1=1.198&r2=1.199)
pgsql/src/backend/optimizer/util:
predtest.c (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/predtest.c.diff?r1=1.2&r2=1.3)