gpu.c: subtree_has_permutable_bands: use isl_schedule_node_every_descendant
gpu.c: subtree_has_permutable_bands: use isl_schedule_node_every_descendant

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

Meinersbur/ppcg,nicolasvasilache/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg