imcc: basic_block 0 in loop

[Steve Fink]

Why is this an error? I think I only encountered this because I had
another bug in my code, but what is wrong with looping back to the
beginning of a subroutine:

    .sub _recurse
            .param int count
            eq count, 0, done
            sub count, 1
            .arg count
            branch _recurse
    done:
            ret
    .end

% ../imcc/imcc loop.imc
error:imcc:find_loops: basic_block 0 in loop