[llvm] [VPlan] Introduce VPSingleDefRecipe. (PR #77023)

[via llvm-commits]

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@@ -819,10 +810,77 @@ class VPRecipeBase : public ilist_node_with_parent<VPRecipeBase, VPBasicBlock>,
   }                                                                            \
   static inline bool classof(const VPRecipeBase *R) {                          \
     return R->getVPDefID() == VPDefID;                                         \
+  }                                                                            \
+  static inline bool classof(const VPSingleDefRecipe *R) {                     \
+    return R->getVPDefID() == VPDefID;                                         \
   }
 
+/// A common base class for recipes defining a single result value.
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ayalz wrote:

The comment documenting VPRecipeBase above should probably move here, and possibly rephrased:
```
/// Single-value
/// VPRecipeBases that also inherit from VPValue must make sure to inherit from
/// VPRecipeBase before VPValue.
```

https://github.com/llvm/llvm-project/pull/77023