isl_union_set_compute_schedule: ignore backward proximity edges on split

Proximity edges are not taken into account during the detection
of strongly connected components.  When we split the dependence graph
based on these SCCs, we may therefore have proximity edges that go
back from a SCC behind the split to a SCC ahead of the split.  These should
be ignored, just like those edges that go from ahead of the split to behind
the split.

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