[scavenger] Remove unused iterator. NFC

Author: darkbuck

llvm/lib/CodeGen/RegisterScavenging.cpp

@@ -300,7 +300,6 @@ Register RegScavenger::scavengeRegisterBackwards(const TargetRegisterClass &RC,
   const MachineFunction &MF = *MBB.getParent();
 
   // Find the register whose use is furthest away.
-  MachineBasicBlock::iterator UseMI;
   ArrayRef<MCPhysReg> AllocationOrder = RC.getRawAllocationOrder(MF);
   std::pair<MCPhysReg, MachineBasicBlock::iterator> P = findSurvivorBackwards(
       *MRI, std::prev(MBBI), To, LiveUnits, AllocationOrder, RestoreAfter);