Make some improvements in the intelligence of the partial-index

predicate tester.  It can now deal with commuted clauses (for
instance, 4 < x implies x > 3), subclauses more complicated than
a simple Var (for example, upper(x) = 't' implies upper(x) > 'a'),
and <> operators (for example, x < 3 implies x <> 4).  Still
only understands operators associated with btree opclasses, though.
Inspired by example from Martin Hampl.