isl_pw_qpolynomial{,_fold}_gist: use simple hull instead of convex hull

isl_pw_qpolynomial{,_fold}_gist: use simple hull instead of convex hull

We need a basic set to gist the domain.  We used to take the convex hull
of the input set, but the convex hull operation can be very slow in isl.
Use the simple hull instead.