[llvm] [VPlan] First step towards VPlan cost modeling (LegacyCM in CostCtx) (PR #92555)

[via llvm-commits]

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@@ -3198,6 +3250,9 @@ class VPlan {
   /// Generate the IR code for this VPlan.
   void execute(VPTransformState *State);
 
+  /// Compute the cost of this plan.
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ayalz wrote:

```suggestion
  /// Return the cost of this plan.
```

https://github.com/llvm/llvm-project/pull/92555