Fix for the following minor issue:
```
error: variable 'first_output_index' set but not used
```

Change-Id: Iccc5e15db62a5768ba6f3742f3c4d7123f493c65
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3652093Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80620}