[turbofan] Robustify the GraphTrimmer.

src/compiler/graph-trimmer.cc

Index: src/compiler/graph-trimmer.cc
diff --git a/src/compiler/graph-trimmer.cc b/src/compiler/graph-trimmer.cc
index 5fae425e1ef248942017a55104b9d800300ab1d2..75071c68b3b5cf1163822edad9aa5f7ca9babaa5 100644
--- a/src/compiler/graph-trimmer.cc
+++ b/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_) {