The existing framework for postra scheduling is library local. We want to keep it that way. Soon we will have a more general MachineScheduler interface. At that time, various bits will be exposed to targets. In the meantime, the VLIWPacketizer wants to use ScheduleDAGInstrs directly, so it needs to wrapped in a PIMPL to avoid exposing it to the target interface.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150633 91177308-0d34-0410-b5e6-96231b3b80d8
