[llvm] [VPlan] Introduce VPlanConstantFolder (PR #125365)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -533,8 +533,8 @@ createScalarIVSteps(VPlan &Plan, InductionDescriptor::InductionKind Kind,
                     VPBuilder &Builder) {
   VPBasicBlock *HeaderVPBB = Plan.getVectorLoopRegion()->getEntryBasicBlock();
   VPCanonicalIVPHIRecipe *CanonicalIV = Plan.getCanonicalIV();
-  VPSingleDefRecipe *BaseIV = Builder.createDerivedIV(
-      Kind, FPBinOp, StartV, CanonicalIV, Step, "offset.idx");
+  VPValue *BaseIV = Builder.createDerivedIV(Kind, FPBinOp, StartV, CanonicalIV,
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artagnon wrote:

See several lines below:

```cpp
    BaseIV = Builder.createScalarCast(Instruction::Trunc, BaseIV, TruncTy, DL);
```

https://github.com/llvm/llvm-project/pull/125365