Add Target hook to duplicate machine instructions.

Some instructions refer to unique labels, and so cannot be trivially cloned
with CloneMachineInstr.

llvm-svn: 92873
diff --git a/llvm/lib/CodeGen/TailDuplication.cpp b/llvm/lib/CodeGen/TailDuplication.cpp
index fddbebd9..c99c74c 100644
--- a/llvm/lib/CodeGen/TailDuplication.cpp
+++ b/llvm/lib/CodeGen/TailDuplication.cpp
@@ -346,7 +346,7 @@
                                      MachineBasicBlock *PredBB,
                                      MachineFunction &MF,
                                      DenseMap<unsigned, unsigned> &LocalVRMap) {
-  MachineInstr *NewMI = MF.CloneMachineInstr(MI);
+  MachineInstr *NewMI = TII->duplicate(MI, MF);
   for (unsigned i = 0, e = NewMI->getNumOperands(); i != e; ++i) {
     MachineOperand &MO = NewMI->getOperand(i);
     if (!MO.isReg())