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

[via llvm-commits]

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@@ -396,7 +398,6 @@ static bool mergeBlocksIntoPredecessors(VPlan &Plan) {
       VPBlockUtils::disconnectBlocks(VPBB, Succ);
       VPBlockUtils::connectBlocks(PredVPBB, Succ);
     }
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ayalz wrote:

```suggestion
    }
    // VPBB is now dead and will be cleaned up when the plan gets destroyed.
```

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