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

[via llvm-commits]

================
@@ -94,6 +94,8 @@ bool isHeaderMask(const VPValue *V, VPlan &Plan);
 /// VPDerivedIV or VPCanonicalIVPHI).
 bool isUniformAcrossVFsAndUFs(VPValue *V);
 
+/// Returns the header block of the top-level vector loop, if one exists.
----------------
ayalz wrote:

```suggestion
/// Returns the header block of the first, top-level loop, or null if none exist.
```

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