[turbofan] Move graph trimming functionality to dedicated 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()) {
+      DCHECK_EQ(IsLive(input),
+                std::find(live_.begin(), live_.end(), input) != live_.end());
+      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();
+      DCHECK_EQ(IsLive(user),
+                std::find(live_.begin(), live_.end(), user) != live_.end());
+      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