[PowerPC] Fixes -Wrange-loop-analysis warnings

[Mordante]

Summary

This avoids new warnings due to D68912 adds -Wrange-loop-analysis to -Wall.

[aaron.ballman]

LGTM with a request to drop top-level consts

llvm/lib/Target/PowerPC/PPCLoopInstrFormPrep.cpp

553

Drop top-level const here and elsewhere?

llvm/lib/Target/PowerPC/PPCLoopInstrFormPrep.cpp

PHINode *NewPHI =
                       Header->getFirstNonPHI());
 
   SCEVExpander SCEVE(*SE, Header->getModule()->getDataLayout(), "pistart");
   Value *BasePtrStart = SCEVE.expandCodeFor(BasePtrStartSCEV, I8PtrTy,
                                             LoopPredecessor->getTerminator());
 
   // Note that LoopPredecessor might occur in the predecessor list multiple
   // times, and we need to add it the right number of times.
-  for (const auto &PI : predecessors(Header)) {
+  for (const auto PI : predecessors(Header)) {
     if (PI != LoopPredecessor)
       continue;
 
     NewPHI->addIncoming(BasePtrStart, LoopPredecessor);
   }
 
   Instruction *PtrInc = nullptr;
   Instruction *NewBasePtr = nullptr;
   if (CanPreInc) {
     Instruction *InsPoint = &*Header->getFirstInsertionPt();
     PtrInc = GetElementPtrInst::Create(
         I8Ty, NewPHI, BasePtrIncSCEV->getValue(),
         getInstrName(MemI, GEPNodeIncNameSuffix), InsPoint);
     cast<GetElementPtrInst>(PtrInc)->setIsInBounds(IsPtrInBounds(BasePtr));
-    for (const auto &PI : predecessors(Header)) {
+    for (const auto PI : predecessors(Header)) {
       if (PI == LoopPredecessor)
         continue;
 
       NewPHI->addIncoming(PtrInc, PI);
     }
     if (PtrInc->getType() != BasePtr->getType())
       NewBasePtr = new BitCastInst(
           PtrInc, BasePtr->getType(),
           getInstrName(PtrInc, CastNodeNameSuffix), InsPoint);
     else
       NewBasePtr = PtrInc;
   } else {
     // Note that LoopPredecessor might occur in the predecessor list multiple
     // times, and we need to make sure no more incoming value for them in PHI.
-    for (const auto &PI : predecessors(Header)) {
+    for (const auto PI : predecessors(Header)) {
       if (PI == LoopPredecessor)
         continue;
 
       // For the latch predecessor, we need to insert a GEP just before the
       // terminator to increase the address.
       BasicBlock *BB = PI;
       Instruction *InsPoint = BB->getTerminator();
       PtrInc = GetElementPtrInst::Create(