isl_tab_basic_map_partial_lexopt: properly normalize divs

isl_tab_basic_map_partial_lexopt: properly normalize divs

We were only dividing out common divisors that also divide
the constant term, but we should ignore the constant term
while determining the greatest common divisor.

We already properly normalize the constraints and not properly
normalizing the divs could result in div constraints not being
recognized as such.  In particular, inside isl_map_affine_hull,
div constraints are added inside the call to isl_basic_map_overlying_set.
Usually, these constraints are removed again in remove_redundant_divs,
but they would remain if the the divs are not properly normalized.

Now, arguably, we shouldn't be adding div constraints inside
isl_map_affine_hull, but a proper normalization of divs is useful in any case.

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