In case the MachineScheduling pass I'm working on doesn't work well
for another target, they can completely override it. This also adds a
hook immediately after the RegAlloc pass to cleanup immediately after
vregs go away. We may want to fold it into the postRA hook later.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@150298 91177308-0d34-0410-b5e6-96231b3b80d8