[turbofan] Robustify the GraphTrimmer.

src/compiler/graph-trimmer.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/graph-trimmer.h"
 
 #include "src/compiler/graph.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 GraphTrimmer::GraphTrimmer(Zone* zone, Graph* graph)
     : graph_(graph), is_live_(graph, 2), live_(zone) {
   live_.reserve(graph->NodeCount());
 }
 
 
 GraphTrimmer::~GraphTrimmer() {}
 
 
 void GraphTrimmer::TrimGraph() {
   // Mark end node as live.
   MarkAsLive(graph()->end());
   // Compute transitive closure of live nodes.
   for (size_t i = 0; i < live_.size(); ++i) {
-    for (Node* const input : live_[i]->inputs()) MarkAsLive(input);
+    Node* const live = live_[i];
+    for (Node* const input : live->inputs()) MarkAsLive(input);
   }
   // Remove dead->live edges.
   for (Node* const live : live_) {
     DCHECK(IsLive(live));
     for (Edge edge : live->use_edges()) {
       Node* const user = edge.from();
       if (!IsLive(user)) {
         if (FLAG_trace_turbo_reduction) {
           OFStream os(stdout);
           os << "DeadLink: " << *user << "(" << edge.index() << ") -> " << *live
              << std::endl;
         }
         edge.UpdateTo(nullptr);
       }
     }
   }
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8