Fix gcc unused-variable warnings after https://crrev.com/c/3706625

Change-Id: I4c9f0315fd0f2cd2cc234007e05234e657ef7e39
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3712422
Auto-Submit: Adam Klein <adamk@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81235}

src/maglev/maglev-compiler.cc

@@ -54,15 +54,19 @@ class NumberingProcessor {
     }
     for (const auto& [index, constant] : graph->root()) {
       constant->set_id(node_id_++);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->smi()) {
       constant->set_id(node_id_++);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->int32()) {
       constant->set_id(node_id_++);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->float64()) {
       constant->set_id(node_id_++);
+      USE(index);
     }
   }
   void PostProcessGraph(MaglevCompilationInfo*, Graph* graph) {}

src/maglev/maglev-vreg-allocator.h

@@ -32,15 +32,19 @@ class MaglevVregAllocator {
     }
     for (const auto& [index, constant] : graph->root()) {
       constant->AllocateVreg(&state_);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->smi()) {
       constant->AllocateVreg(&state_);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->int32()) {
       constant->AllocateVreg(&state_);
+      USE(index);
     }
     for (const auto& [index, constant] : graph->float64()) {
       constant->AllocateVreg(&state_);
+      USE(index);
     }
   }
   void PostProcessGraph(MaglevCompilationInfo*, Graph* graph) {