isl_map_simplify.c: is_opposite: return isl_bool
isl_map_simplify.c: is_opposite: return isl_bool

This clarifies what the possible return values are.

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

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