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

[via llvm-commits]

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@@ -514,6 +546,11 @@ void VPBasicBlock::execute(VPTransformState *State) {
 
   // 2. Fill the IR basic block with IR instructions.
   executeRecipes(State, NewBB);
+
+  // If this block is a latch, update CurrentParentLoop.
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ayalz wrote:

```suggestion
  // If this block is a latch, update CurrentParentLoop. The second successor of a latch is a header, with the other successor being an exit/middle.
```
Wrapping in `VPBlockUtils::isLatch()` may be clearer?

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