target/i386: Trivial code motion and code style fix

A new pair of braces has to be added to declare variables in the case block.
The code style is also fixed according to the transalte.c itself during the
code motion.

Signed-off-by: Ziqiao Kong <ziqiaokong@gmail.com>
Message-Id: <20210530150112.74411-1-ziqiaokong@gmail.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>