isl_map_simplify.c: extract out shared constraint index handling
isl_map_simplify.c: extract out shared constraint index handling

In particular, extract out the shared code in
isl_basic_map_remove_duplicate_constraints and remove_shifted_constraints.

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

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