linux-user: Relax single exit from "break"

Transform outermost "break" to "return ret".  If the immediately
preceeding statement was an assignment to ret, return the value
directly.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180818190118.12911-4-richard.henderson@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>