[MachinePiepliner] Don't check boundary node in checkValidNodeOrder

This was exposed by PowerPC target enablement.

In ScheduleDAG, if we haven't seen any uses in this scheduling region,
we will create a dependence edge to ExitSU to model the live-out latency.
This is required for vreg defs with no in-region use, and prefetches with
no vreg def.

When we build NodeOrder in Scheduler, we ignore these boundary nodes.
However, when we check Succs in checkValidNodeOrder, we did not skip
them, so we still assume all the nodes have been sorted and in order in
Indices array. So when we call lower_bound() for ExitSU, it will return
Indices.end(), causing memory issues in following Node access.

Differential Revision: https://reviews.llvm.org/D63282

llvm-svn: 363329