target/loongarch: Implement vaddi/vsubi

This patch includes:
- VADDI.{B/H/W/D}U;
- VSUBI.{B/H/W/D}U.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230504122810.4094787-6-gaosong@loongson.cn>