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

[Florian Hahn via llvm-commits]

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@@ -1166,6 +1166,30 @@ 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);
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fhahn wrote:

I don't think we could  `dyn_cast` from `VPPhiAccessors` to `VPRecipeBase` because there's no directy relationship between them (and the `this` pointer for the `VPPhiAccessors` (sub-)object may not be the same as the VPRecipeBase (base-) object. The other way works, because we can down-cast to the concrete types inheriting from `VPPhiAccessors`).

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