isl_qpolynomial_is_equal: also compare spaces and divs

isl_qpolynomial_is_equal: also compare spaces and divs

Before, isl_qpolynomial_is_equal would only consider the "shape"
of the polynomial, meaning that two polynomial living in different
spaces could be considered equal.  Worse, expressions in terms
of different integer divisions could also be considered equal.
We now take into account both space and divs so that we no longer
produce false positives.

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