Fix library visibility problems with VLIWPacketizer.

Fix library visibility problems with VLIWPacketizer.

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