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

[via llvm-commits]

================
@@ -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);
----------------
ayalz wrote:

Hmm, would having getAsRecipe() do a dyn_cast to VPRecipeBase be ok? Possibly followed by an assert...

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