isl_pw_qpolynomial_involves_dims: also check whether domains involve dims

isl_pw_qpolynomial_involves_dims: also check whether domains involve dims

Before, isl_pw_qpolynomial_involves_dims would only check whether the
polynomials associated to the different cells involved any of the indicated
dimensions, but didn't check if the decriptions of the cells involved
any of them.
If isl_pw_qpolynomial_involves_dims is used to check whether
isl_pw_qpolynomial_drop_dims can safely be applied, then clearly we need
to test the cell constraints too.

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