Index: include/llvm/Support/GenericDomTree.h
===================================================================
--- include/llvm/Support/GenericDomTree.h
+++ include/llvm/Support/GenericDomTree.h
@@ -666,10 +666,30 @@
   /// splitBlock - BB is split and now it has one successor. Update dominator
   /// tree to reflect this change.
   void splitBlock(NodeT *NewBB) {
-    if (IsPostDominator)
-      Split<Inverse<NodeT *>>(NewBB);
-    else
+    if (!IsPostDominator)
       Split<NodeT *>(NewBB);
+    else {
+      // FIXME: Split(NewBB) only works for dominator trees. For post-dominator
+      // trees we just uses applyUpdates and so is potentially less efficient
+      // than it could be.
+      using GraphT = GraphTraits<NodeT *>;
+      assert(std::distance(GraphT::child_begin(NewBB),
+                           GraphT::child_end(NewBB)) == 1 &&
+             "NewBB should have a single successor!");
+      NodeT *OldBB = *GraphT::child_begin(NewBB);
+
+      std::vector<UpdateType> Updates;
+      SmallPtrSet<NodeT *, 4> Visited;
+      for (auto *Pred : children<Inverse<NodeT *>>(NewBB)) {
+        if (!Visited.insert(Pred).second)
+          // Make sure our updates and balanced, even if Pred is a child
+          // multiple times
+          continue;
+        Updates.emplace_back(UpdateKind::Insert, Pred, NewBB);
+        Updates.emplace_back(UpdateKind::Delete, Pred, OldBB);
+      }
+      applyUpdates(Updates);
+    }
   }
 
   /// print - Convert to human readable form