Fix compilation with gcc

gcc version 8.4 is currently throwing the following compilation
error:
```
 error: variable 'first' set but not used
```

Change-Id: I25e1197b8deaa0089adee2e736448976705c7497
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3602060Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#80153}

src/compiler/turboshaft/operations.h

@@ -398,6 +398,7 @@ struct OperationT : Operation {
                                  std::index_sequence<I...>) {
     os << "[";
     bool first = true;
+    USE(first);
     ((first ? (first = false, os << std::get<I>(options))
             : os << ", " << std::get<I>(options)),
      ...);