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

[Alexey Bataev via llvm-commits]

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@@ -7763,7 +7758,7 @@ DenseMap<const SCEV *, Value *> LoopVectorizationPlanner::executePlan(
                                       LLVMLoopVectorizeFollowupVectorized});
 
   VPBasicBlock *HeaderVPBB =
-      BestVPlan.getVectorLoopRegion()->getEntryBasicBlock();
+      cast<VPBasicBlock>(BestVPlan.getVectorPreheader()->getSingleSuccessor());
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alexey-bataev wrote:

```suggestion
  auto *HeaderVPBB =
      cast<VPBasicBlock>(BestVPlan.getVectorPreheader()->getSingleSuccessor());
```

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