[llvm] [VPlan] Model epilogue skeleton in VPlan by wrapping in VPIRBBs (NFCI). (PR #222312)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1461,6 +1429,58 @@ static void insertCheckBlockBeforeVectorLoop(VPlan &Plan,
   addIncomingForLastPredecessor(ScalarPH);
 }
 
+void VPlanTransforms::modelGeneratedMainLoopBlocks(
+    VPlan &Plan, VPlan &MainPlan, VPIRBasicBlock *EnteredFrom) {
+  // Map blocks from MainPlan to new, empty VPIRBasicBlocks in Plan, so the
+  // skeleton CFG can be modeled explicitly. MainPlan's entry maps to Plan's
+  // now-disconnected entry, its scalar PH to EnteredFrom, mapped last so it can
+  // be dropped as source below. Blocks without successors have no edges.
+  auto *MainEntry = cast<VPIRBasicBlock>(MainPlan.getEntry());
+  auto *MainScalarPH = cast<VPIRBasicBlock>(MainPlan.getScalarPreheader());
+  SmallVector<VPIRBasicBlock *> BypassBlocks;
+  SmallMapVector<VPIRBasicBlock *, VPIRBasicBlock *, 8> Old2NewVPBB;
+  Old2NewVPBB[MainEntry] = cast<VPIRBasicBlock>(Plan.getEntry());
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
+      MainEntry);
+  for (VPIRBasicBlock *VPBB : VPBlockUtils::blocksAs<VPIRBasicBlock>(RPOT)) {
+    // Collect bypass blocks (minimum iteration checks, runtime checks).
+    if (VPBB->getNumSuccessors() == 2 &&
+        VPBB->getSuccessors()[0] == MainScalarPH)
+      BypassBlocks.push_back(VPBB);
+    if (VPBB != MainEntry && VPBB != MainScalarPH && VPBB->hasSuccessors())
+      Old2NewVPBB[VPBB] =
+          Plan.createEmptyVPIRBasicBlock(VPBB->getIRBasicBlock());
+  }
+  Old2NewVPBB[MainScalarPH] = EnteredFrom;
+
+  // First, connect the edges from the bypass blocks to the scalar preheader, in
+  // reverse order, to preserve the predecessor order of the generated IR. The
+  // last bypass block branches into Plan and is mirrored below.
+  VPBasicBlock *ScalarPH = Plan.getScalarPreheader();
----------------
artagnon wrote:

```suggestion
  VPBasicBlock *EpiScalarPH = Plan.getScalarPreheader();
```

I think we should call it MainPlan and EpiPlan, and not MainPlan and Plan, which is confusing?

https://github.com/llvm/llvm-project/pull/222312