[llvm] [VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (PR #114292)

[via llvm-commits]

================
@@ -2502,14 +2522,15 @@ void InnerLoopVectorizer::emitIterationCountCheck(BasicBlock *Bypass) {
                                DT->getNode(Bypass)->getIDom()) &&
          "TC check is expected to dominate Bypass");
 
-  // Update dominator for Bypass & LoopExit (if needed).
-  DT->changeImmediateDominator(Bypass, TCCheckBlock);
   BranchInst &BI =
       *BranchInst::Create(Bypass, LoopVectorPreHeader, CheckMinIters);
   if (hasBranchWeightMD(*OrigLoop->getLoopLatch()->getTerminator()))
     setBranchWeights(BI, MinItersBypassWeights, /*IsExpected=*/false);
   ReplaceInstWithInst(TCCheckBlock->getTerminator(), &BI);
   LoopBypassBlocks.push_back(TCCheckBlock);
+
+  // TODO: Wrap LoopVectorPreHeader in VPIRBasicBlock here.
+  introduceCheckBlockInVPlan(Plan, nullptr);
----------------
ayalz wrote:

```suggestion
  introduceCheckBlockInVPlan(Plan, TCCheckBlock);
```
There is a check block, but it is introduced by fusing its conditional branch into Entry, rather than having a separate BB to be wrapped it in a VPIRBB.

https://github.com/llvm/llvm-project/pull/114292