isl_schedule_constraints_compute_schedule: produce schedule tree based schedule

isl_schedule_constraints_compute_schedule: produce schedule tree based schedule

By constructing a schedule tree directly, we no longer need to keep
track of the extra band data that would allow us to extract a band
forest from the schedule graph nodes.

Since schedule trees will also be used for other purposes, we now
explicitly keep track of whether a band is permutable.  In particular,
we mark those bands that were constructed using the Pluto like
scheduler as permutable, even if they contain only a single dimension.
If we have to resort to any Feautrier style scheduling then
these bands are not marked permutable.

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