[turbofan] Robustify the GraphTrimmer.

src/compiler/graph-trimmer.h

 // 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.
 
 #ifndef V8_COMPILER_GRAPH_TRIMMER_H_
 #define V8_COMPILER_GRAPH_TRIMMER_H_
 
 #include "src/compiler/node-marker.h"
 
 namespace v8 {
@@ skipping 10 matching lines @@
   GraphTrimmer(Zone* zone, Graph* graph);
   ~GraphTrimmer();
 
   // Trim nodes in the {graph} that are not reachable from {graph->end()}.
   void TrimGraph();
 
   // Trim nodes in the {graph} that are not reachable from either {graph->end()}
   // or any of the roots in the sequence [{begin},{end}[.
   template <typename ForwardIterator>
   void TrimGraph(ForwardIterator begin, ForwardIterator end) {
-    while (begin != end) MarkAsLive(*begin++);
+    while (begin != end) {
+      Node* const node = *begin++;
+      if (!node->IsDead()) MarkAsLive(node);
+    }
     TrimGraph();
   }
 
  private:
   V8_INLINE bool IsLive(Node* const node) { return is_live_.Get(node); }
   V8_INLINE void MarkAsLive(Node* const node) {
-    if (!node->IsDead() && !IsLive(node)) {
+    DCHECK(!node->IsDead());
+    if (!IsLive(node)) {
       is_live_.Set(node, true);
       live_.push_back(node);
     }
   }
 
   Graph* graph() const { return graph_; }
 
   Graph* const graph_;
   NodeMarker<bool> is_live_;
   NodeVector live_;
 
   DISALLOW_COPY_AND_ASSIGN(GraphTrimmer);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_GRAPH_TRIMMER_H_