advice on minor cleanups?

[Zubin Duggal]

Looking at the code, it seems like it does return an actual `Linkable`
when we are in interpreted mode:

runHscBackendPhase pipe_env hsc_env mod_name src_flavour location result = do
     ...
         -> case backend dflags of
           Interpreter -> do
             ...
             return ([], final_iface, Just linkable, panic "interpreter")