[NFC][LoopUtils] Use RewritePhi::ExpansionPoint for Inst to be replaced.

[HsiangKai]

Summary

The instruction to be replaced is recorded in RewritePhi::ExpansionPoint
when collecting RewritePhiSet. We could use the field directly. Besides
this, move Inst closer to its usage.

[mkazantsev]

Any tests showing the difference?

[HsiangKai]

In D106250#2933594, @mkazantsev wrote:

Any tests showing the difference?

There should be no difference for the modification.
In LoopUtils.cpp:1337, the structure RewritePhi is initialized. PN->getIncomingValue(Phi.Ith) should be equal to Phi.ExpansionPoint.

[reames]

Personally, I find the existing code much easier to read than the new code. Patch rejected, unless you can justify why this is useful.

llvm/lib/Transforms/Utils/LoopUtils.cpp

for (const RewritePhi &Phi : RewritePhiSet) {
     // Only do the rewrite when the ExitValue can be expanded cheaply.
     // If LoopCanBeDel is true, rewrite exit value aggressively.
     if (ReplaceExitValue == OnlyCheapRepl && !LoopCanBeDel && Phi.HighCost) {
       DeadInsts.push_back(ExitVal);
       continue;
     }
 
     NumReplaced++;
-    Instruction *Inst = cast<Instruction>(PN->getIncomingValue(Phi.Ith));
     PN->setIncomingValue(Phi.Ith, ExitVal);
 
     // If this instruction is dead now, delete it. Don't do it now to avoid
     // invalidating iterators.
+    Instruction *Inst = Phi.ExpansionPoint;
     if (isInstructionTriviallyDead(Inst, TLI))
       DeadInsts.push_back(Inst);
 
     // Replace PN with ExitVal if that is legal and does not break LCSSA.
     if (PN->getNumIncomingValues() == 1 &&
         LI->replacementPreservesLCSSAForm(PN, ExitVal)) {
       PN->replaceAllUsesWith(ExitVal);
       PN->eraseFromParent();