arm/translate-a64: mark path as unreachable to eliminate warning

Fixes the following warning when compiling with gcc 5.4.0 with -O1
optimizations and --enable-debug:

target/arm/translate-a64.c: In function ‘aarch64_tr_translate_insn’:
target/arm/translate-a64.c:2361:8: error: ‘post_index’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if (!post_index) {
        ^
target/arm/translate-a64.c:2307:10: note: ‘post_index’ was declared here
     bool post_index;
          ^
target/arm/translate-a64.c:2386:8: error: ‘writeback’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if (writeback) {
        ^
target/arm/translate-a64.c:2308:10: note: ‘writeback’ was declared here
     bool writeback;
          ^

Note that idx comes from selecting 2 bits, and therefore its value
can be at most 3.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1510087611-1851-1-git-send-email-cota@braap.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
