[VPlan] Add VPBlockUtils::transferSuccessors (NFCI).

Add a new helper to transfer successors to a new, unconnected VPBB.
Helps to simplify existing code, and prepare for upcoming changes.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -843,15 +843,14 @@ void VPRegionBlock::dissolveToCFGLoop() {
 
   VPBlockBase *Preheader = getSinglePredecessor();
   auto *ExitingLatch = cast<VPBasicBlock>(getExiting());
-  VPBlockBase *Middle = getSingleSuccessor();
+
   VPBlockUtils::disconnectBlocks(Preheader, this);
-  VPBlockUtils::disconnectBlocks(this, Middle);
 
   for (VPBlockBase *VPB : vp_depth_first_shallow(Entry))
     VPB->setParent(getParent());
 
   VPBlockUtils::connectBlocks(Preheader, Header);
-  VPBlockUtils::connectBlocks(ExitingLatch, Middle);
+  VPBlockUtils::transferSuccessors(this, ExitingLatch);
   VPBlockUtils::connectBlocks(ExitingLatch, Header);
 }
 

llvm/lib/Transforms/Vectorize/VPlanUtils.h

@@ -116,12 +116,7 @@ class VPBlockUtils {
            NewBlock->getPredecessors().empty() &&
            "Can't insert new block with predecessors or successors.");
     NewBlock->setParent(BlockPtr->getParent());
-    SmallVector<VPBlockBase *> Succs(BlockPtr->successors());
-    for (VPBlockBase *Succ : Succs) {
-      Succ->replacePredecessor(BlockPtr, NewBlock);
-      NewBlock->appendSuccessor(Succ);
-    }
-    BlockPtr->clearSuccessors();
+    transferSuccessors(BlockPtr, NewBlock);
     connectBlocks(BlockPtr, NewBlock);
   }
 
@@ -205,6 +200,14 @@ class VPBlockUtils {
     Old->clearSuccessors();
   }
 
+  /// Transfer successors from \p Old to \p New. \p New must have no successors.
+  static void transferSuccessors(VPBlockBase *Old, VPBlockBase *New) {
+    for (auto *Succ : Old->getSuccessors())
+      Succ->replacePredecessor(Old, New);
+    New->setSuccessors(Old->getSuccessors());
+    Old->clearSuccessors();
+  }
+
   /// Return an iterator range over \p Range which only includes \p BlockTy
   /// blocks. The accesses are casted to \p BlockTy.
   template <typename BlockTy, typename T>