[llvm] [VPlan] Connect Entry to scalar preheader during initial construction. (PR #140132)

[via llvm-commits]

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@@ -7876,7 +7870,10 @@ EpilogueVectorizerMainLoop::emitIterationCountCheck(BasicBlock *Bypass,
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
 
-  introduceCheckBlockInVPlan(TCCheckBlock);
+  // Only generate add a new block for the trip-count check for the main loop.
+  // The epilogue will use the already existing VPlan entry block.
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ayalz wrote:

```suggestion
  // When vectorizing the main loop, its trip-count check is placed in a new block, whereas the overall trip-count check is placed in the VPlan entry block.
  // When vectorizing the epilogue loop, its trip-count check is placed in the VPlan entry block.
```
sounds clearer? Referring to https://llvm.org/docs/Vectorizers.html#epilogue-vectorization

https://github.com/llvm/llvm-project/pull/140132