isl_polynomial.c: reduce_divs: use isl_qpolynomial_get_ctx
isl_polynomial.c: reduce_divs: use isl_qpolynomial_get_ctx

This is a minor clean-up in preparation for a fix of reduce_divs.

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

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