[IndVars] Extract out eliminateIdentitySCEV, NFC

[sanjoy]

Summary

Reflow a comment while at it.

I'm not actually looking for pre-commit review on this change, this is
here just to provide context for later changes.

Having said that, feel free to review if you wish.

llvm/trunk/lib/Transforms/Utils/SimplifyIndVar.cpp

public:
 
     /// Iteratively perform simplification on a worklist of users of the
     /// specified induction variable. This is the top-level driver that applies
     /// all simplifications to users of an IV.
     void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
 
     Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
 
+    bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
+
     bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
     void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
     void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
                               bool IsSigned);
     bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
 
     Instruction *splitOverflowIntrinsic(Instruction *IVUser,
                                         const DominatorTree *DT);
void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
   }
 
   DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
   ++NumElimRem;
   Changed = true;
   DeadInsts.emplace_back(Rem);
 }
 
-/// Eliminate an operation that consumes a simple IV and has
-/// no observable side-effect given the range of IV values.
-/// IVOperand is guaranteed SCEVable, but UseInst may not be.
+/// Eliminate an operation that consumes a simple IV and has no observable
+/// side-effect given the range of IV values.  IVOperand is guaranteed SCEVable,
+/// but UseInst may not be.
 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
                                      Instruction *IVOperand) {
   if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
     eliminateIVComparison(ICmp, IVOperand);
     return true;
   }
   if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
     bool IsSigned = Rem->getOpcode() == Instruction::SRem;
     if (IsSigned || Rem->getOpcode() == Instruction::URem) {
       eliminateIVRemainder(Rem, IVOperand, IsSigned);
       return true;
     }
   }
 
-  // Eliminate any operation that SCEV can prove is an identity function.
+  if (eliminateIdentitySCEV(UseInst, IVOperand))
+    return true;
+
+  return false;
+}
+
+/// Eliminate any operation that SCEV can prove is an identity function.
+bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
+                                           Instruction *IVOperand) {
   if (!SE->isSCEVable(UseInst->getType()) ||
       (UseInst->getType() != IVOperand->getType()) ||
       (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
     return false;
 
   DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
 
   UseInst->replaceAllUsesWith(IVOperand);