[PowerPC] Use a small cleanup pass to remove VSX self copies

As explained in r204976, because of how the allocation of VSX registers
interacts with the call-lowering code, we sometimes end up generating self VSX
copies. Specifically, things like this:
  %VSL2<def> = COPY %F2, %VSL2<imp-use,kill>
(where %F2 is really a sub-register of %VSL2, and so this copy is a nop)

This adds a small cleanup pass to remove these prior to post-RA scheduling.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204980 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCTargetMachine.cpp

@@ -182,6 +182,9 @@ bool PPCPassConfig::addPreRegAlloc() {
 }
 
 bool PPCPassConfig::addPreSched2() {
+  if (getPPCSubtarget().hasVSX())
+    addPass(createPPCVSXCopyCleanupPass());
+
   if (getOptLevel() != CodeGenOpt::None)
     addPass(&IfConverterID);