Simplify box_overlap computations.

Given the assumption that a box's high coordinates are not less than its
low coordinates, the tests in box_ov() are overly complicated and can be
reduced to about half as much work.  Since many other functions in
geo_ops.c rely on that assumption, there doesn't seem to be a good reason
not to use it here.

Per discussion of Alexander Korotkov's GiST fix, which was already using
the simplified logic (in a non-fuzzy form, but the equivalence holds just
as well for fuzzy).