isl_schedule_constraints_compute_schedule: further clean-up
isl_schedule_constraints_compute_schedule: further clean-up

Now that we have extracted graph_init from
isl_schedule_constraints_compute_schedule, we can avoid looking
into the internals of the isl_schedule_constraints inside
isl_schedule_constraints_compute_schedule.

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

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