[gcc] Silence some more unused var warnings

Change-Id: Id3586eae6580416216bce2c448e6885aaa1589b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732111
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81430}

src/maglev/maglev-regalloc.cc

@@ -239,15 +239,19 @@ void StraightForwardRegisterAllocator::AllocateRegisters() {
   }
   for (const auto& [index, constant] : graph_->root()) {
     constant->SetConstantLocation();
+    USE(index);
   }
   for (const auto& [value, constant] : graph_->smi()) {
     constant->SetConstantLocation();
+    USE(value);
   }
   for (const auto& [value, constant] : graph_->int32()) {
     constant->SetConstantLocation();
+    USE(value);
   }
   for (const auto& [value, constant] : graph_->float64()) {
     constant->SetConstantLocation();
+    USE(value);
   }
 
   for (block_it_ = graph_->begin(); block_it_ != graph_->end(); ++block_it_) {