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

[Zoltan Somogyi]

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);


Zoltan.
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