[llvm] [VPlan] Implement cloning of VPlans. (PR #73158)

[Alexey Bataev via llvm-commits]

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@@ -2536,6 +2695,9 @@ class VPlan {
   /// been modeled in VPlan directly.
   DenseMap<const SCEV *, VPValue *> SCEVToExpansion;
 
+  /// Construct an uninitialized VPlan, should be used for cloning only.
+  VPlan() {}
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alexey-bataev wrote:

```suggestion
  explicit VPlan() = default;
```

https://github.com/llvm/llvm-project/pull/73158