[turbofan] Robustify the GraphTrimmer.

src/compiler/graph-trimmer.h

Index: src/compiler/graph-trimmer.h
diff --git a/src/compiler/graph-trimmer.h b/src/compiler/graph-trimmer.h
index d8258becc8d6e4c3cb46ac9bdbc50a9ae28ab460..98d335a44df22eba6db95c7dad2a356869dfdaf6 100644
--- a/src/compiler/graph-trimmer.h
+++ b/src/compiler/graph-trimmer.h
@@ -28,14 +28,18 @@ class GraphTrimmer final {
   // 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);
     }