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

[via llvm-commits]

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@@ -2428,6 +2428,32 @@ InnerLoopVectorizer::getOrCreateVectorTripCount(BasicBlock *InsertBlock) {
   return VectorTripCount;
 }
 
+/// Helper to connect both the vector and scalar preheaders to the Plan's
+/// entry. This is used when adjusting \p Plan during skeleton
+/// creation, i.e. adjusting the plan after introducing an initial runtime
+/// check.
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ayalz wrote:

```suggestion
/// Helper to connect the scalar preheader directly to the Plan's entry, as a bypass of the vector loop. This adjusts \p Plan during skeleton creation after introducing an initial runtime check at Entry's end. The scalar preheader is initially connected to the middle block only, as a remainder of the vector loop.
```

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