[NFC] fix diagnostic

It's pretty silly to diagnose on a scalar copy but the build does that:
  loop variable 'SibReg' of type 'const llvm::Register' creates a copy from type 'const llvm::Register' [-Wrange-loop-analysis]

Author: jfbastien

llvm/lib/CodeGen/InlineSpiller.cpp

@@ -1197,7 +1197,7 @@ bool HoistSpillHelper::isSpillCandBB(LiveInterval &OrigLI, VNInfo &OrigVNI,
   SmallSetVector<Register, 16> &Siblings = Virt2SiblingsMap[OrigReg];
   assert(OrigLI.getVNInfoAt(Idx) == &OrigVNI && "Unexpected VNI");
 
-  for (auto const SibReg : Siblings) {
+  for (const Register &SibReg : Siblings) {
     LiveInterval &LI = LIS.getInterval(SibReg);
     VNInfo *VNI = LI.getVNInfoAt(Idx);
     if (VNI) {