Fix oversight in indexscan plan creation. I recently added code to use

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.