[llvm] [VPlan] Use InstSimplifyFolder instead of TargetFolder (PR #141222)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

The issue is that we have this VPlan:

```
<x1> vector loop: {
  vector.body:
    EMIT vp<%6> = CANONICAL-INDUCTION ir<0>, vp<%index.next>
  Successor(s): pred.sdiv

  <xVFxUF> pred.sdiv: {
    pred.sdiv.entry:
      BRANCH-ON-MASK vp<%5>
    Successor(s): pred.sdiv.if, pred.sdiv.continue

    pred.sdiv.if:
      REPLICATE ir<%sdiv> = sdiv ir<%sext.0>, ir<%zext.true> (S->V)
    Successor(s): pred.sdiv.continue

    pred.sdiv.continue:
      PHI-PREDICATED-INSTRUCTION vp<%7> = ir<%sdiv>
    No successors
  }
```

where `REPLICATE ir<%sdiv> = sdiv ir<%sext.0>, ir<%zext.true>` gets simplified to the Value `%sext.0`, and RAUW changes the PredPHI to `PHI-PREDICATED-INSTRUCTION vp<%7> = ir<%sext.0>`: this PredPHI would have got simplified with TargetFolder before this commit as follows.

```cpp
  if (auto *PredPHI = dyn_cast<VPPredInstPHIRecipe>(&R)) {
    VPlan *Plan = R.getParent()->getPlan();
    VPValue *Op = PredPHI->getOperand(0);
    if (!Op->isLiveIn() || !Op->getLiveInIRValue())
      return;
    if (auto *C = dyn_cast<Constant>(Op->getLiveInIRValue()))
      PredPHI->replaceAllUsesWith(Plan->getOrAddLiveIn(C));
  }
```

However, the match against a Constant fails, and the PredPHI isn't simplified. This should be an easy fix, which I will post shortly.

https://github.com/llvm/llvm-project/pull/141222