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

[Florian Hahn via llvm-commits]

================
@@ -1844,23 +1854,30 @@ class VPWidenInductionRecipe : public VPHeaderPHIRecipe {
 class VPWidenIntOrFpInductionRecipe : public VPWidenInductionRecipe {
   TruncInst *Trunc;
 
+  bool isUnrolled() const { return getNumOperands() == 6; }
+
 public:
   VPWidenIntOrFpInductionRecipe(PHINode *IV, VPValue *Start, VPValue *Step,
-                                VPValue *VF, const InductionDescriptor &IndDesc,
-                                DebugLoc DL)
+                                VPValue *VF, VPValue *StepVector,
+                                const InductionDescriptor &IndDesc, DebugLoc DL)
       : VPWidenInductionRecipe(VPDef::VPWidenIntOrFpInductionSC, IV, Start,
                                Step, IndDesc, DL),
         Trunc(nullptr) {
     addOperand(VF);
+    // Dummy StepVector replaced in convertToConcreteRecipes
+    addOperand(StepVector);
   }
 
   VPWidenIntOrFpInductionRecipe(PHINode *IV, VPValue *Start, VPValue *Step,
-                                VPValue *VF, const InductionDescriptor &IndDesc,
+                                VPValue *VF, VPValue *StepVector,
+                                const InductionDescriptor &IndDesc,
                                 TruncInst *Trunc, DebugLoc DL)
       : VPWidenInductionRecipe(VPDef::VPWidenIntOrFpInductionSC, IV, Start,
                                Step, IndDesc, DL),
         Trunc(Trunc) {
     addOperand(VF);
+    // Dummy StepVector replaced in convertToConcreteRecipes
----------------
fhahn wrote:

Comment doesn't seem accurate now, as we pass in `StepVector`. Could we still create the poison value here and maybe say something like `Temporarily use Poison for step-vector, which will only be introduced when needed, when preparing to execute.`

Also, should we exclude it from printing, if poison?

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