Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -597,23 +597,28 @@
   // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
   // the instruction.
   while (!I.use_empty()) {
-    // The user must be a PHI node.
-    PHINode *PN = cast<PHINode>(I.user_back());
-
-    BasicBlock *ExitBlock = PN->getParent();
-    assert(ExitBlockSet.count(ExitBlock) &&
-           "The LCSSA PHI is not in an exit block!");
-
-    Instruction *New;
-    auto It = SunkCopies.find(ExitBlock);
-    if (It != SunkCopies.end())
-      New = It->second;
-    else
-      New = SunkCopies[ExitBlock] =
-          CloneInstructionInExitBlock(I, *ExitBlock, *PN);
-
-    PN->replaceAllUsesWith(New);
-    PN->eraseFromParent();
+    Instruction *User = I.user_back();
+    if (DT->isReachableFromEntry(User->getParent())) {
+      // The user must be a PHI node.
+      PHINode *PN = cast<PHINode>(User);
+
+      BasicBlock *ExitBlock = PN->getParent();
+      assert(ExitBlockSet.count(ExitBlock) &&
+             "The LCSSA PHI is not in an exit block!");
+
+      Instruction *New;
+      auto It = SunkCopies.find(ExitBlock);
+      if (It != SunkCopies.end())
+        New = It->second;
+      else
+        New = SunkCopies[ExitBlock] =
+            CloneInstructionInExitBlock(I, *ExitBlock, *PN);
+
+      PN->replaceAllUsesWith(New);
+      PN->eraseFromParent();
+    } else {
+      User->replaceUsesOfWith(&I, UndefValue::get(I.getType()));
+    }
   }
 
   CurAST->deleteValue(&I);
Index: test/Transforms/LICM/PR19798.ll
===================================================================
--- /dev/null
+++ test/Transforms/LICM/PR19798.ll
@@ -0,0 +1,22 @@
+; RUN: opt -licm -S < %s | FileCheck %s
+
+define void @f() {
+; CHECK-LABEL: @f(
+entry:
+  br label %bb0
+
+bb0:
+  %tobool7 = icmp eq i1 undef, undef
+  br label %bb1
+
+bb1:
+  br i1 undef, label %bb0, label %bb0
+
+unreachable:
+; CHECK-LABEL: unreachable:
+; CHECK:   br i1 undef, label %unreachable, label %unreachable
+  br i1 %tobool7, label %unreachable, label %unreachable
+
+bb3:
+  unreachable
+}