[m-rev.] diff: fix unreachable return statements

[Julien Fischer]

On Fri, 1 Sep 2017, Zoltan Somogyi wrote:

> This fixes a bootcheck failure in Java grades, but the Java grade
> still does not bootstrap. The problem is shown by the following
> messages, which implicate Julien's recent changes.
>
> Making Mercury/javas/jmercury/transform_hlds__ctgc__structure_reuse__direct__detect_garbage.java
> Making Java class files
> Mercury/javas/jmercury/ll_backend__lookup_switch.java:390: error: incompatible types: possible lossy conversion from int to short
>    MR_scalar_common_27[0] = new ll_backend__llds.Rval_const_0.Llconst_int16_1(0);
>                                                                               ^
> Mercury/javas/jmercury/ll_backend__lookup_switch.java:394: error: incompatible types: possible lossy conversion from int to byte
>    MR_scalar_common_29[0] = new ll_backend__llds.Rval_const_0.Llconst_int8_1(0);
>                                                                              ^
> Mercury/javas/jmercury/ll_backend__lookup_switch.java:398: error: incompatible types: possible lossy conversion from int to short
>    MR_scalar_common_31[0] = new ll_backend__llds.Rval_const_0.Llconst_uint16_1(0);
>
> ^
> Mercury/javas/jmercury/ll_backend__lookup_switch.java:402: error: incompatible types: possible lossy conversion from int to byte
>    MR_scalar_common_33[0] = new ll_backend__llds.Rval_const_0.Llconst_uint8_1(0);

I have committed a fix for the above problem.

Julien.