target/i386: silence the compiler warnings in gen_shiftd_rm_T1

target/i386: silence the compiler warnings in gen_shiftd_rm_T1

The current "#ifdef TARGET_X86_64" statement affects
the compiler's determination of fall through.

When using -Wimplicit-fallthrough in our CFLAGS, the compiler showed warning:
target/i386/translate.c: In function ‘gen_shiftd_rm_T1’:
target/i386/translate.c:1773:12: warning: this statement may fall through [-Wimplicit-fallthrough=]
         if (is_right) {
            ^
target/i386/translate.c:1782:5: note: here
     case MO_32:
     ^~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201211152426.350966-6-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>