Index: lib/IR/ConstantFold.cpp
===================================================================
--- lib/IR/ConstantFold.cpp
+++ lib/IR/ConstantFold.cpp
@@ -998,9 +998,16 @@
         return C1;
       // undef << X -> 0
       return Constant::getNullValue(C1->getType());
+    default:
+      // We don't know how to handle UNDEFs in this instruction.
+      return nullptr;
     }
   }
 
+  // At this point neither constant should be an UndefValue.
+  assert(!isa<UndefValue>(C1) && !isa<UndefValue>(C2) &&
+         "Unexpected UndefValue");
+
   // Handle simplifications when the RHS is a constant int.
   if (ConstantInt *CI2 = dyn_cast<ConstantInt>(C2)) {
     switch (Opcode) {
@@ -1102,7 +1109,6 @@
       return ConstantExpr::get(Opcode, C2, C1);
   }
 
-  // At this point we know neither constant is an UndefValue.
   if (ConstantInt *CI1 = dyn_cast<ConstantInt>(C1)) {
     if (ConstantInt *CI2 = dyn_cast<ConstantInt>(C2)) {
       const APInt &C1V = CI1->getValue();