target/loongarch: Fix [X]VLDI raising exception incorrectly

target/loongarch: Fix [X]VLDI raising exception incorrectly

According to the specification, [X]VLDI should trigger an invalid instruction
exception only when Bit[12] is 1 and Bit[11:8] > 12. This patch fixes an issue
where an exception was incorrectly raised even when Bit[12] was 0.

Test case:

```
    .global main
main:
    vldi    $vr0, 3328
    ret
```

Reported-by: Zhou Qiankang <wszqkzqk@qq.com>
Signed-off-by: WANG Rui <wangrui@loongson.cn>
Reviewed-by: Song Gao <gaosong@loongson.cn>
Message-ID: <20250804132212.4816-1-wangrui@loongson.cn>
Signed-off-by: Song Gao <gaosong@loongson.cn>