[turbofan] Implement simplistic checkpoint reducer.

test/unittests/compiler/checkpoint-elimination-unittest.cc

+// Copyright 2016 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/checkpoint-elimination.h"
+#include "src/compiler/common-operator.h"
+#include "src/compiler/operator.h"
+#include "test/unittests/compiler/graph-reducer-unittest.h"
+#include "test/unittests/compiler/graph-unittest.h"
+#include "test/unittests/compiler/node-test-utils.h"
+
+using testing::StrictMock;
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+class CheckpointEliminationTest : public GraphTest {
+ public:
+  CheckpointEliminationTest() : GraphTest() {}
+  ~CheckpointEliminationTest() override {}
+
+ protected:
+  Reduction Reduce(AdvancedReducer::Editor* editor, Node* node) {
+    CheckpointElimination reducer(editor);
+    return reducer.Reduce(node);
+  }
+
+  Reduction Reduce(Node* node) {
+    StrictMock<MockAdvancedReducerEditor> editor;
+    return Reduce(&editor, node);
+  }
+};
+
+namespace {
+
+const Operator kOpNoWrite(0, Operator::kNoWrite, "OpNoWrite", 0, 1, 0, 0, 1, 0);
+
+}  // namespace
+
+// -----------------------------------------------------------------------------
+// CheckPoint
+
+TEST_F(CheckpointEliminationTest, CheckPointChain) {
+  Node* const control = graph()->start();
+  Node* frame_state = EmptyFrameState();
+  Node* checkpoint1 = graph()->NewNode(common()->CheckPoint(), frame_state,
+                                       graph()->start(), control);
+  Node* effect_link = graph()->NewNode(&kOpNoWrite, checkpoint1);
+  Node* checkpoint2 = graph()->NewNode(common()->CheckPoint(), frame_state,
+                                       effect_link, control);
+  Reduction r = Reduce(checkpoint2);
+  ASSERT_TRUE(r.Changed());
+  EXPECT_EQ(effect_link, r.replacement());
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8