[turbofan] Introduce CommonOperatorReducer.

src/compiler/common-operator-reducer.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/common-operator-reducer.h"
+
+#include "src/compiler/common-operator.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+Reduction CommonOperatorReducer::Reduce(Node* node) {
+  switch (node->opcode()) {
+    case IrOpcode::kEffectPhi:
+    case IrOpcode::kPhi: {
+      int const input_count = node->InputCount();
+      if (input_count > 1) {
+        Node* const replacement = node->InputAt(0);
+        for (int i = 1; i < input_count - 1; ++i) {
+          if (node->InputAt(i) != replacement) return NoChange();
+        }
+        return Replace(replacement);
+      }
+      break;
+    }
+    case IrOpcode::kSelect: {
+      if (node->InputAt(1) == node->InputAt(2)) {
+        return Replace(node->InputAt(1));
+      }
+      break;
+    }
+    default:
+      break;
+  }
+  return NoChange();
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8