isl_coalesce.c: check_adj_eq: extract out check_single_adj_eq
isl_coalesce.c: check_adj_eq: extract out check_single_adj_eq

This will make it easier to extend check_single_adj_eq
in a subsequent commit.

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

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