Index: llvm/lib/Analysis/CFG.cpp
===================================================================
--- llvm/lib/Analysis/CFG.cpp
+++ llvm/lib/Analysis/CFG.cpp
@@ -189,14 +189,23 @@
       // All blocks in a single loop are reachable from all other blocks. From
       // any of these blocks, we can skip directly to the exits of the loop,
       // ignoring any other blocks inside the loop body.
+      Visited.insert(Outer->block_begin(), Outer->block_end());
       Outer->getExitBlocks(Worklist);
     } else if (DTN) {
-      // The dominance check effectively visited all blocks dominated by BB.
-      // Skip over the descendants of the DomTreeNode to visit their successors.
-      for (auto I = df_begin(DTN), E = df_end(DTN); I != E; ++I) {
-        for (auto Succ : successors(I->getBlock()))
+      // The dominance check effectively visits all blocks dominated by BB. Skip
+      // over the domtree-descendants of the block to visit their successors.
+      for (auto I = df_begin(DTN), E = df_end(DTN); I != E;) {
+        for (auto Succ : successors(I->getBlock())) {
           if (!DT->dominates(DTN, DT->getNode(Succ)))
             Worklist.push_back(Succ);
+        }
+        ++I;
+        while (I != E && !Visited.insert(I->getBlock()).second) {
+          // Don't enqueue any domtree-descendants of a visited block. We've
+          // already either visited its descendants or enqueued them.
+          // I.skipChildren() implicitly performs ++I.
+          I.skipChildren();
+        }
       }
     } else {
       Worklist.append(succ_begin(BB), succ_end(BB));