target/loongarch: Replace CHECK_SXE to check_vec(ctx, 16)

Introduce a new function check_vec to replace CHECK_SXE

Signed-off-by: Song Gao <gaosong@loongson.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230914022645.1151356-11-gaosong@loongson.cn>