Some variables are only used in DCHECKs, which leading to the
`unused-but-set-variable` check fail in release mode.
So we need to explicitly mark them as used.

Change-Id: I18dc8ec97ea5fbbaa081a52629b9572b863d901e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3237558Reviewed-by: Liu yu <liuyu@loongson.cn>
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Cr-Commit-Position: refs/heads/main@{#77507}