isl_schedule_node_has_parent: reuse isl_schedule_node_get_tree_depth
isl_schedule_node_has_parent: reuse isl_schedule_node_get_tree_depth

This removes some (minor) code duplication.

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

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl