pn.cc: determine_dependence_properties: only set "scheduled" if we actually have

pn.cc: determine_dependence_properties: only set "scheduled" if we actually have

Before this commit, we would turn on "scheduled", even if we hadn't
actually scheduled anything.  This happens to work in most cases
because either we don't need to schedule or the first time we
go through that code, we have actually scheduled (that is, we are
dealing with an external dependence).

The buggy code was introduced in 85bf545 (pn: merge iterations over
dependences for determining properties, Tue Sep 6 09:57:41 2011 +0200)
and resulted in an incorrect network for the odd_even test case when
the ordering of dependences was changed in 6bf746b (update isl for change
in access sorting in isl_access_info_compute_flow,
Thu Apr 5 16:51:07 2012 +0200).

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