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

[via llvm-commits]

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@@ -618,6 +630,9 @@ class VPBlockBase {
   /// VPBlockBase, thereby "executing" the VPlan.
   virtual void execute(VPTransformState *State) = 0;
 
+  /// Compute the cost of the block.
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ayalz wrote:

```suggestion
  /// Return the cost of the block.
```

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