Index: lib/Transforms/Utils/CloneFunction.cpp
===================================================================
--- lib/Transforms/Utils/CloneFunction.cpp
+++ lib/Transforms/Utils/CloneFunction.cpp
@@ -325,10 +325,11 @@
       // the basic block.
       if (Value *V =
               SimplifyInstruction(NewInst, BB->getModule()->getDataLayout())) {
-        // On the off-chance that this simplifies to an instruction in the old
-        // function, map it back into the new function.
-        if (Value *MappedV = VMap.lookup(V))
-          V = MappedV;
+        assert((!isa<Instruction>(V) ||
+                dyn_cast<Instruction>(V)->getParent() == nullptr ||
+                dyn_cast<Instruction>(V)->getFunction() != OldFunc ||
+                OldFunc == NewFunc) &&
+               "Simplified Instruction should not be in the old function.");
 
         if (!NewInst->mayHaveSideEffects()) {
           VMap[&*II] = V;