Index: llvm/trunk/lib/Transforms/Scalar/LICM.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Scalar/LICM.cpp
+++ llvm/trunk/lib/Transforms/Scalar/LICM.cpp
@@ -597,8 +597,13 @@
   // PHI nodes in exit blocks due to LCSSA form. Just RAUW them with clones of
   // the instruction.
   while (!I.use_empty()) {
+    Instruction *User = I.user_back();
+    if (!DT->isReachableFromEntry(User->getParent())) {
+      User->replaceUsesOfWith(&I, UndefValue::get(I.getType()));
+      continue;
+    }
     // The user must be a PHI node.
-    PHINode *PN = cast<PHINode>(I.user_back());
+    PHINode *PN = cast<PHINode>(User);
 
     BasicBlock *ExitBlock = PN->getParent();
     assert(ExitBlockSet.count(ExitBlock) &&
Index: llvm/trunk/test/Transforms/LICM/PR19798.ll
===================================================================
--- llvm/trunk/test/Transforms/LICM/PR19798.ll
+++ llvm/trunk/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
+}