isl_map_simplify.c: extract out setup_constraint_index
isl_map_simplify.c: extract out setup_constraint_index

This will be reused for removing shifted constraints represented by a matrix.

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

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