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

[via llvm-commits]

================
@@ -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
+/// preheader.
+static void introduceCheckBlockInVPlan(VPlan &Plan, BasicBlock *CheckIRBB) {
+  VPBlockBase *ScalarPH = Plan.getScalarPreheader();
+  VPBlockBase *VectorPH = Plan.getVectorPreheader();
+  VPBlockBase *PreVectorPH = VectorPH->getSinglePredecessor();
+  assert(PreVectorPH->getSuccessors()[0] == ScalarPH && "Unexpected successor");
+  VPIRBasicBlock *CheckVPIRBB = VPIRBasicBlock::fromBasicBlock(CheckIRBB);
+  VPBlockUtils::connectBlocks(CheckVPIRBB, ScalarPH);
+  VPBlockUtils::insertOnEdge(PreVectorPH, VectorPH, CheckVPIRBB);
----------------
ayalz wrote:

```suggestion
  // Connect ScalarPH before VectorPH (via insert on edge), so that ScalarPH is the first successor of the new check block.
  VPBlockUtils::connectBlocks(CheckVPIRBB, ScalarPH);
  VPBlockUtils::insertOnEdge(PreVectorPH, VectorPH, CheckVPIRBB);
```
or first insert on edge, so that check block has vector loop preheader as its only successor, similar to the initial Entry block, and then connect it also to scalar loop preheader, followed by swapping the successors, aligning with the above.

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