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

BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
 import("//build/module_args/v8.gni")
 
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
@@ skipping 545 matching lines @@
     "src/compiler/move-optimizer.h",
     "src/compiler/node-aux-data-inl.h",
     "src/compiler/node-aux-data.h",
     "src/compiler/node-cache.cc",
     "src/compiler/node-cache.h",
     "src/compiler/node-matchers.h",
     "src/compiler/node-properties-inl.h",
     "src/compiler/node-properties.h",
     "src/compiler/node.cc",
     "src/compiler/node.h",
+    "src/compiler/opcodes.cc",
     "src/compiler/opcodes.h",
     "src/compiler/operator-properties.cc",
     "src/compiler/operator-properties.h",
     "src/compiler/operator.cc",
     "src/compiler/operator.h",
     "src/compiler/pipeline.cc",
     "src/compiler/pipeline.h",
     "src/compiler/pipeline-statistics.cc",
     "src/compiler/pipeline-statistics.h",
     "src/compiler/raw-machine-assembler.cc",
@@ skipping 881 matching lines @@
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
     ]
   }
 
   direct_dependent_configs = [ ":external_config" ]
 }
 
 }