[llvm] [VPlan] Replace VPRegionBlock with explicit CFG before execute (NFCI). (PR #117506)

[via llvm-commits]

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@@ -207,6 +207,29 @@ VPBlockBase *VPBlockBase::getEnclosingBlockWithPredecessors() {
   return Parent->getEnclosingBlockWithPredecessors();
 }
 
+bool VPBlockUtils::isHeader(const VPBlockBase *VPB,
+                            const VPDominatorTree &VPDT) {
+  auto *VPBB = dyn_cast<VPBasicBlock>(VPB);
+  if (!VPBB)
+    return false;
+
+  // If VPBB is in a region R, VPBB is a loop header if R is a loop region with
+  // VPBB as its entry, i.e., free of predecessors.
+  if (auto *R = VPBB->getParent())
+    return !R->isReplicator() && VPBB->getNumPredecessors() == 0;
+
+  // A header dominates its second predecessor (the latch), with the other
+  // predecessor being the preheader
+  return VPB->getPredecessors().size() == 2 &&
+         VPDT.dominates(VPB, VPB->getPredecessors()[1]);
+}
+
+bool VPBlockUtils::isLatch(const VPBlockBase *VPB,
+                           const VPDominatorTree &VPDT) {
+  return VPB->getNumSuccessors() == 2 &&
----------------
ayalz wrote:

```suggestion
  // A latch has a header as its second successor, with its other successor leaving the loop.
  // A preheader OTOH has a header as its first (and only) successor.
  return VPB->getNumSuccessors() == 2 &&
```

https://github.com/llvm/llvm-project/pull/117506