isl_map_simplify.c: coalesce_divs: simplify based on fix in previous commit
isl_map_simplify.c: coalesce_divs: simplify based on fix in previous commit

The previous commit ensures that the coefficient of the other
existentially quantified variable that is involved in the coalescing
is always 1 or -1 in the constraints of the input to coalesce_divs.
Simplify coalesce_divs taking into account this information.

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

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