I think I had gotten confused and mixed up "undefined variables" (Python
variables which hadn't gotten set) and "undefined values" (results of
expressions that the JIT knows will throw, or otherwise can't evaluate).

For undefined *variables*, we represent them using None, since they can
still have refcount operations done on them.

For undefined *values* (or results), we can use llvm's undefValues, since
they should never be touched.