[turbofan] Turn IrOpcode::Mnemonic() into a table lookup.

src/compiler/opcodes.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/opcodes.h"
+
+#include <algorithm>
+
+#include "src/base/macros.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+namespace {
+
+char const* const kMnemonics[] = {
+#define DECLARE_MNEMONIC(x) #x,
+    ALL_OP_LIST(DECLARE_MNEMONIC)
+#undef DECLARE_MNEMONIC
+        "UnknownOpcode"};
+
+}  // namespace
+
+
+// static
+char const* IrOpcode::Mnemonic(Value value) {
+  size_t const n = std::max<size_t>(value, arraysize(kMnemonics) - 1);
+  return kMnemonics[n];
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8