[llvm] [VPlan] Add VPInstruction::StepVector and use it in VPWidenIntOrFpInductionRecipe (PR #129508)

[Luke Lau via llvm-commits]

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@@ -1965,8 +1965,11 @@ void VPWidenIntOrFpInductionRecipe::execute(VPTransformState &State) {
   }
 
   Value *SplatStart = Builder.CreateVectorSplat(State.VF, Start);
-  Value *SteppedStart = getStepVector(SplatStart, Step, ID.getInductionOpcode(),
-                                      State.VF, State.Builder);
+  assert(cast<VPInstruction>(getStepVector())->getOpcode() ==
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lukel97 wrote:

Fixed vectors would still be a VPInstruction::StepVector. Hopefully the need for this assertion would also go away if we fully expand the recipe in #118638 

https://github.com/llvm/llvm-project/pull/129508