[turbofan] Robustify the GraphTrimmer.

The GraphTrimmer should not ever see a dead node, except for the roots
that are explicitly fed into it. To defend against this, turn the
condition into a DCHECK.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1698883003

Cr-Commit-Position: refs/heads/master@{#34001}

src/compiler/graph-trimmer.cc

@@ -24,7 +24,8 @@ void GraphTrimmer::TrimGraph() {
   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_) {

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);
     }