[llvm] [VPlan] Introduce VPlan-level constant folder (PR #125365)

[Florian Hahn via llvm-commits]

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@@ -938,10 +939,87 @@ static void recursivelyDeleteDeadRecipes(VPValue *V) {
   }
 }
 
+/// Try to fold \p R using TargetFolder to a constant. Will succeed for a
+/// handled \p Opcode if all \p Operands are constant.
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fhahn wrote:

```suggestion
/// Try to fold \p R using TargetFolder to a constant. Will succeed and return true if \p Opcode is supported and all \p Operands are constant.
```

https://github.com/llvm/llvm-project/pull/125365