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

[Luke Lau via llvm-commits]

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@@ -1094,6 +1093,12 @@ void VPInstructionWithType::execute(VPTransformState &State) {
     State.set(this, Cast, VPLane(0));
     break;
   }
+  case VPInstruction::StepVector: {
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lukel97 wrote:

I think it's currently costed as zero for now since we don't want to change the overall cost of VPWidenIntOrFpInductionRecipe. 

Once we expand VPWidenIntOrFpInductionRecipe in #118638 I think it should be safe to add a cost to it then? Since the expansion will happen just before execution, after any costing. 

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