[llvm] [VPlan] Replace UnrollPart for VPScalarIVSteps with start index op (NFC) (PR #170906)

[via llvm-commits]

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@@ -3876,6 +3872,8 @@ class LLVM_ABI_FOR_TEST VPScalarIVStepsRecipe : public VPRecipeWithIRFlags,
 
   VPValue *getStepValue() const { return getOperand(1); }
 
+  VPValue *getVFValue() const { return getOperand(2); }
+
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ayalz wrote:

```suggestion
  /// Return the number of scalars to produce per unroll part, used to compute StartIndex during unrolling.
  VPValue *getVFValue() const { return getOperand(2); }

  /// Return the StartIndex, or null if known to be zero, valid only after unrolling.
  VPValue *getStartIndex() const { return getNumOperands() == 4 ? getOperand(3) : nullptr; }

```

https://github.com/llvm/llvm-project/pull/170906