[turbofan] Introduce separate SelectLowering reducer.

test/unittests/compiler/select-lowering-unittest.cc

+// Copyright 2014 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/select-lowering.h"
+#include "test/unittests/compiler/graph-unittest.h"
+#include "test/unittests/compiler/node-test-utils.h"
+#include "testing/gmock-support.h"
+
+using testing::AllOf;
+using testing::Capture;
+using testing::CaptureEq;
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+class SelectLoweringTest : public GraphTest {
+ public:
+  SelectLoweringTest() : GraphTest(5), lowering_(graph(), common()) {}
+
+ protected:
+  Reduction Reduce(Node* node) { return lowering_.Reduce(node); }
+
+ private:
+  SelectLowering lowering_;
+};
+
+
+TEST_F(SelectLoweringTest, SelectWithSameConditions) {
+  Node* const p0 = Parameter(0);
+  Node* const p1 = Parameter(1);
+  Node* const p2 = Parameter(2);
+  Node* const p3 = Parameter(3);
+  Node* const p4 = Parameter(4);
+
+  Capture<Node*> branch;
+  Capture<Node*> merge;
+  {
+    Reduction const r =
+        Reduce(graph()->NewNode(common()->Select(kMachInt32), p0, p1, p2));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(
+        r.replacement(),
+        IsPhi(
+            kMachInt32, p1, p2,
+            AllOf(CaptureEq(&merge),
+                  IsMerge(IsIfTrue(CaptureEq(&branch)),
+                          IsIfFalse(AllOf(CaptureEq(&branch),
+                                          IsBranch(p0, graph()->start())))))));
+  }
+  {
+    Reduction const r =
+        Reduce(graph()->NewNode(common()->Select(kMachInt32), p0, p3, p4));
+    ASSERT_TRUE(r.Changed());
+    EXPECT_THAT(r.replacement(), IsPhi(kMachInt32, p3, p4, CaptureEq(&merge)));
+  }
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8