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.