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

[via llvm-commits]

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@@ -2760,6 +2760,15 @@ LoopVectorizationCostModel::getVectorIntrinsicCost(CallInst *CI,
   return TTI.getIntrinsicInstrCost(CostAttrs, CostKind);
 }
 
+static VPBasicBlock *getHeaderForMainVectorLoop(VPlan &Plan,
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ayalz wrote:

```suggestion
static VPBasicBlock *getSingleLoopHeader(VPlan &Plan,
```
? As this is regardless of loop being vector, main or otherwise. I.e., works until epilog and/or scalar remainder/fallback loops join main vector loop in same Plan.

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