Fix compilation error with gcc

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}

src/compiler/turboshaft/optimization-phase.h

@@ -149,6 +149,7 @@ struct OptimizationPhase<Analyzer, Assembler>::Impl {
         const Operation& op = *it;
         OpIndex index = it.Index();
         OpIndex first_output_index = assembler.graph().next_operation_index();
+        USE(first_output_index);
         if constexpr (trace_reduction) TraceReductionStart(index);
         if (!analyzer.OpIsUsed(index)) {
           if constexpr (trace_reduction) TraceOperationUnused();