[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[via llvm-commits]

================
@@ -14,26 +14,57 @@
 #include "LoopVectorizationPlanner.h"
 #include "VPlan.h"
 #include "VPlanCFG.h"
+#include "VPlanDominatorTree.h"
 #include "VPlanTransforms.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 
 using namespace llvm;
 
+/// Create and return a new VPRegionBlock for loop starting at \p HeaderVPBB, if
+/// it is a header of a loop.
+static VPRegionBlock *introduceRegion(VPlan &Plan, VPBlockBase *HeaderVPBB,
+                                      VPDominatorTree &VPDT) {
+  if (HeaderVPBB->getNumPredecessors() != 2)
+    return nullptr;
+  VPBlockBase *PreheaderVPBB = HeaderVPBB->getPredecessors()[0];
+  VPBlockBase *LatchVPBB = HeaderVPBB->getPredecessors()[1];
+  if (!VPDT.dominates(HeaderVPBB, LatchVPBB))
+    return nullptr;
+  assert(VPDT.dominates(PreheaderVPBB, HeaderVPBB) &&
+         "preheader must dominate header");
+  VPBlockUtils::disconnectBlocks(PreheaderVPBB, HeaderVPBB);
+  VPBlockUtils::disconnectBlocks(LatchVPBB, HeaderVPBB);
+  VPBlockBase *Succ = LatchVPBB->getSingleSuccessor();
+  if (Succ)
+    VPBlockUtils::disconnectBlocks(LatchVPBB, Succ);
+
+  auto *R = Plan.createVPRegionBlock(HeaderVPBB, LatchVPBB, "",
+                                     false /*isReplicator*/);
+  // All VPBB's reachable shallowly from HeaderVPBB belong to top level loop,
+  // because VPlan is expected to end at top level latch.
----------------
ayalz wrote:

```suggestion
  // because VPlan is expected to end at top level latch disconnected above.
```


https://github.com/llvm/llvm-project/pull/129402