isl_basic_map_overlying_set: simply reset space when no divs are involved
isl_basic_map_overlying_set: simply reset space when no divs are involved

In particular, there is no need to call simplify (again) if no
integer divisions are getting introduced.

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

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