[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.
+/// TODO: Connect scalar preheader during initial VPlan construction.
+static void connectScalarPreheaderAsBypassInVPlan(VPlan &Plan) {
+  VPBlockBase *VectorPH = Plan.getVectorPreheader();
+  VPBlockBase *ScalarPH = Plan.getScalarPreheader();
+  VPBlockBase *Entry = Plan.getEntry();
+  VPBlockUtils::connectBlocks(Entry, ScalarPH);
+  std::swap(Entry->getSuccessors()[0], Entry->getSuccessors()[1]);
+}
+
+/// Introduces a new VPIRBasicBlock for \p CheckIRBB to \p Plan between the
+/// vector preheader and its predecessor, also connecting to the scalar
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ayalz wrote:

```suggestion
/// vector preheader and its predecessor, also connecting the new block to the scalar
```

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