[llvm] [VPlan] Add VPPhiAccessors to provide interface for phi recipes (NFC) (PR #129388)

[Florian Hahn via llvm-commits]

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@@ -1171,6 +1171,59 @@ class VPIRInstruction : public VPRecipeBase {
   void extractLastLaneOfFirstOperand(VPBuilder &Builder);
 };
 
+/// Helper type to provide functions to access incoming values and blocks for
+/// phi-like recipes. RecipeTy must be a sub-class of VPRecipeBase.
+template <typename RecipeTy> class VPPhiAccessors {
+  /// Return a VPRecipeBase* to the current object.
+  const VPRecipeBase *getAsRecipe() const {
+    return static_cast<const RecipeTy *>(this);
+  }
+
+public:
+  /// Returns the incoming VPValue with index \p Idx.
+  VPValue *getIncomingValue(unsigned Idx) const {
+    return getAsRecipe()->getOperand(Idx);
+  }
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fhahn wrote:

we need it for VPWidenPHIRecipe, VPHeaderPHIRecipe, VPIRPhi, VPEVLBasedPhi and VPPhi (scalar phis via VPInstruction, probably via a new specialization).

Define both `getIncomingBlock` and `getIncomingValue` there

https://github.com/llvm/llvm-project/pull/129388