[llvm] [VPlan] Replace VPRegionBlock with explicit CFG before execute (NFCI). (PR #117506)

[via llvm-commits]

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@@ -124,3 +125,12 @@ bool vputils::isUniformAcrossVFsAndUFs(VPValue *V) {
         return false;
       });
 }
+
+VPBasicBlock *vputils::getTopLevelVectorLoopHeader(VPlan &Plan,
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ayalz wrote:

```suggestion
VPBasicBlock *vputils::getFirstLoopHeader(VPlan &Plan,
```
as it is agnostic to loop being vector or not, can also be FirstTopLevel but First implies TopLevel?

https://github.com/llvm/llvm-project/pull/117506