[llvm] [VPlan] Manage created blocks directly in VPlan. (NFC) (PR #120918)

[via llvm-commits]

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@@ -1217,6 +1222,7 @@ static void remapOperands(VPBlockBase *Entry, VPBlockBase *NewEntry,
 }
 
 VPlan *VPlan::duplicate() {
+  unsigned CreatedBlockSize = CreatedBlocks.size();
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ayalz wrote:

```suggestion
  unsigned NumBlocksBeforeCloning = CreatedBlocks.size();
```

https://github.com/llvm/llvm-project/pull/120918