isl_polynomial.c: reduce_divs: do not assume input is a monomial
isl_polynomial.c: reduce_divs: do not assume input is a monomial

Even though the original intention may have been to only call
reduce_divs on a monomial, there is no guarantee that some
of the intermediate functions that end up calling reduce_divs
aren't called on other expressions.
Furthermore, the recursive call inside reduce_divs itself
is not guaranteed to have a monomial argument.
Fix these potential issues by not assuming that the input is a monomial.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

Meinersbur/isl,UBERTC/isl,inducer/isl-mirror,tobig/isl,tobig/isl,UBERTC/isl,nicolasvasilache/isl,PollyLabs/isl,Meinersbur/isl,PollyLabs/isl,nicolasvasilache/isl,tobig/isl,UBERTC/isl,PollyLabs/isl,PollyLabs/isl,nicolasvasilache/isl,Meinersbur/isl,inducer/isl-mirror,inducer/isl-mirror,nicolasvasilache/isl,inducer/isl-mirror,PollyLabs/isl,tobig/isl,inducer/isl-mirror,Meinersbur/isl,nicolasvasilache/isl,UBERTC/isl