isl_map_simplify.c: check_for_div_constraints: also check for "better" ones

isl_map_simplify.c: check_for_div_constraints: also check for "better" ones

In check_for_div_constraints we check if two "opposite" constraints
can be used to define a div.  However, a given div may be definable
by multiple pairs of constraints and then we would just pick the pair
we come across.

We now also consider possible defining constraints for divs for which
we already have a definition, which makes the result less dependent
on the order of the constraints.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>