isl_output.c: print_constraints: extract out constraint_op
isl_output.c: print_constraints: extract out constraint_op

This will be reused in the next commit.

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

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