isl_map_plain_is_disjoint: extract out all_pairs
isl_map_plain_is_disjoint: extract out all_pairs

This function will be reused to optimize isl_map_is_disjoint
in an upcoming commit.

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

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