isl_scheduler.c: update_edge: return isl_stat
isl_scheduler.c: update_edge: return isl_stat

This clarifies what the possible return values are.

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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