Index: lib/Transforms/Utils/LoopSimplify.cpp
===================================================================
--- lib/Transforms/Utils/LoopSimplify.cpp
+++ lib/Transforms/Utils/LoopSimplify.cpp
@@ -339,6 +339,41 @@
     }
   }
 
+  if (PreserveLCSSA) {
+    // Fix LCSSA form for L. Some values, which previously were only used inside
+    // L, can now be used in NewOuter loop. We need to insert phi-nodes for them
+    // in corresponding exit blocks.
+    SmallVector<BasicBlock *, 8> ExitBlocks;
+    L->getExitBlocks(ExitBlocks);
+    for (Instruction &I : *NewBB) {
+      PHINode *PN = dyn_cast<PHINode>(&I);
+      if (!PN)
+        break;
+      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
+        Instruction *V = dyn_cast<Instruction>(PN->getIncomingValue(i));
+        if (!V || !L->contains(V))
+          continue;
+
+        // We've found a value from L used in OuterL. Let's find a spot where
+        // we need to insert a phi node to preserve LCSSA form.
+        BasicBlock *IncomingBB = PN->getIncomingBlock(i);
+        for (BasicBlock *ExitBB : ExitBlocks) {
+          if (!DT->dominates(ExitBB, IncomingBB))
+            continue;
+
+          // We've found the exit block where we need to insert the phi-node.
+          // Create and start using it instead of previous def.
+          PHINode *NewPN = PHINode::Create(
+              PN->getType(), 1, V->getName() + ".lcssa", &ExitBB->front());
+          for (BasicBlock *PredBB : predecessors(ExitBB))
+            NewPN->addIncoming(V, PredBB);
+          PN->setIncomingValue(i, NewPN);
+          break;
+        }
+      }
+    }
+  }
+
   return NewOuter;
 }
 
Index: test/Transforms/LoopSimplify/pr28272.ll
===================================================================
--- /dev/null
+++ test/Transforms/LoopSimplify/pr28272.ll
@@ -0,0 +1,31 @@
+; RUN: opt < %s -lcssa -loop-unroll -S | FileCheck %s
+target triple = "x86_64-unknown-linux-gnu"
+
+; Check that we don't crash on this test.
+; LoopSimplify is invoked by LoopUnroll.
+; CHECK-LABEL: @foo
+define void @foo() {
+entry:
+  br label %bb29
+
+bb29:
+  br label %bb40
+
+bb40:
+  br i1 true, label %bb40, label %bb43
+
+bb43:
+  %a = phi i32 [ undef, %bb40 ], [ 0, %bb45 ], [ %a, %bb54 ]
+  %b = phi i32 [ 0, %bb40 ], [ 1, %bb54 ], [ %c, %bb45 ]
+  br i1 true, label %bb114, label %bb29
+
+bb114:
+  %c = add i32 0, 1
+  br i1 true, label %bb45, label %bb54
+
+bb45:
+  br label %bb43
+
+bb54:
+  br label %bb43
+}