[VPlan] Factor out isUnrolled() helper in VPWidenIntOrFpInductionRecipe. NFC (#137635)

Split off from #129508, this generalizes getSplatVFValue and
getLastUnrolledPartOperand so they don't need changed if another operand
is added.

Author: lukel97

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1860,6 +1860,9 @@ class VPWidenInductionRecipe : public VPHeaderPHIRecipe {
 class VPWidenIntOrFpInductionRecipe : public VPWidenInductionRecipe {
   TruncInst *Trunc;
 
+  // If this recipe is unrolled it will have 2 additional operands.
+  bool isUnrolled() const { return getNumOperands() == 5; }
+
 public:
   VPWidenIntOrFpInductionRecipe(PHINode *IV, VPValue *Start, VPValue *Step,
                                 VPValue *VF, const InductionDescriptor &IndDesc,
@@ -1908,9 +1911,9 @@ class VPWidenIntOrFpInductionRecipe : public VPWidenInductionRecipe {
   const VPValue *getVFValue() const { return getOperand(2); }
 
   VPValue *getSplatVFValue() {
-    // If the recipe has been unrolled (4 operands), return the VPValue for the
-    // induction increment.
-    return getNumOperands() == 5 ? getOperand(3) : nullptr;
+    // If the recipe has been unrolled return the VPValue for the induction
+    // increment.
+    return isUnrolled() ? getOperand(getNumOperands() - 2) : nullptr;
   }
 
   /// Returns the first defined value as TruncInst, if it is one or nullptr
@@ -1932,7 +1935,7 @@ class VPWidenIntOrFpInductionRecipe : public VPWidenInductionRecipe {
   /// the last unrolled part, if it exists. Returns itself if unrolling did not
   /// take place.
   VPValue *getLastUnrolledPartOperand() {
-    return getNumOperands() == 5 ? getOperand(4) : this;
+    return isUnrolled() ? getOperand(getNumOperands() - 1) : this;
   }
 };