target/loongarch: Implement xvaddi/xvsubi

This patch includes:
- XVADDI.{B/H/W/D}U;
- XVSUBI.{B/H/W/D}U.

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-17-gaosong@loongson.cn>