[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.

llvm-svn: 204980
diff --git a/llvm/lib/Target/PowerPC/PPC.h b/llvm/lib/Target/PowerPC/PPC.h
index df8dd8f..c42c5be 100644
--- a/llvm/lib/Target/PowerPC/PPC.h
+++ b/llvm/lib/Target/PowerPC/PPC.h
@@ -37,6 +37,7 @@
 #endif
   FunctionPass *createPPCEarlyReturnPass();
   FunctionPass *createPPCVSXCopyPass();
+  FunctionPass *createPPCVSXCopyCleanupPass();
   FunctionPass *createPPCVSXFMAMutatePass();
   FunctionPass *createPPCBranchSelectionPass();
   FunctionPass *createPPCISelDag(PPCTargetMachine &TM);