[llvm] [VPlan] Remove Def pointer from for VPSingleDefRecipes (NFC) (PR #195483)

[via llvm-commits]

================
@@ -305,28 +307,63 @@ struct VPSymbolicValue : public VPValue {
   bool Materialized = false;
 };
 
-/// A VPValue defined by a recipe that produces one or more values.
+/// Abstract base class for VPValues defined by a VPRecipeBase.
 class VPRecipeValue : public VPValue {
   friend class VPValue;
   friend class VPDef;
 
-  /// Pointer to the VPRecipeBase that defines this VPValue.
-  VPRecipeBase *Def;
-
 #if !defined(NDEBUG)
   /// Returns true if this VPRecipeValue is defined by \p D.
   /// NOTE: Only used by VPDef to assert that VPRecipeValues added/removed from
   /// /p D are associated with its VPRecipeBase,
   bool isDefinedBy(const VPDef *D) const;
 #endif
 
+protected:
+  VPRecipeValue(unsigned char SC, Value *UV = nullptr) : VPValue(SC, UV) {}
+
+public:
+  LLVM_ABI_FOR_TEST virtual ~VPRecipeValue() = 0;
+
+  static bool classof(const VPValue *V) {
+    return V->getVPValueID() == VPVStandaloneRecipeValueSC ||
+           V->getVPValueID() == VPVSingleDefValueSC;
+  }
+};
+
+/// A VPRecipeValue embedded as a subobject of VPSingleDefRecipe.
+class VPSingleDefValue : public VPRecipeValue {
+  friend class VPDef;
+  friend class VPSingleDefRecipe;
+
+protected:
+  /// Construct a VPSingleDefValue. Must only be used by VPSingleDefRecipe.
+  LLVM_ABI_FOR_TEST VPSingleDefValue(VPSingleDefRecipe *Def,
+                                     Value *UV = nullptr);
+
+public:
+  static bool classof(const VPValue *V) {
+    return V->getVPValueID() == VPVSingleDefValueSC;
+  }
+};
+
+/// A VPRecipeValue that stores a pointer to its defining recipe.
+class VPStandaloneRecipeValue : public VPRecipeValue {
+  friend class VPDef;
+
+  /// Pointer to the VPRecipeBase that defines this VPValue.
----------------
ayalz wrote:

```suggestion
  /// Pointer to the multi-def recipe that defines this VPValue, among others.
```

https://github.com/llvm/llvm-project/pull/195483