target/loongarch: Implement vneg

This patch includes;
- VNEG.{B/H/W/D}.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Song Gao <gaosong@loongson.cn>
Message-Id: <20230504122810.4094787-7-gaosong@loongson.cn>
3 files changed