a test case to remind us to support undef on the LHS

I intended it as a test case for undef on the LHS, we would need to change the test case once we can infer undef on the RHS as well. But I can see that Souper could (once we have undef on the LHS) correctly infer 0 on the RHS, making this not a good test case for the intended purpose. I'll modify it, thanks.