[turbofan] Optimize string "length" property access based on types.

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.
 
 import("//build/config/android/config.gni")
 import("//build/config/arm.gni")
 import("//build/config/mips.gni")
 
 # Because standalone V8 builds are not supported, assume this is part of a
 # Chromium build.
@@ skipping 669 matching lines @@
     "src/compiler/js-graph.cc",
     "src/compiler/js-graph.h",
     "src/compiler/js-inlining.cc",
     "src/compiler/js-inlining.h",
     "src/compiler/js-intrinsic-lowering.cc",
     "src/compiler/js-intrinsic-lowering.h",
     "src/compiler/js-operator.cc",
     "src/compiler/js-operator.h",
     "src/compiler/js-type-feedback.cc",
     "src/compiler/js-type-feedback.h",
+    "src/compiler/js-type-feedback-lowering.cc",
+    "src/compiler/js-type-feedback-lowering.h",
     "src/compiler/js-typed-lowering.cc",
     "src/compiler/js-typed-lowering.h",
     "src/compiler/jump-threading.cc",
     "src/compiler/jump-threading.h",
     "src/compiler/linkage-impl.h",
     "src/compiler/linkage.cc",
     "src/compiler/linkage.h",
     "src/compiler/liveness-analyzer.cc",
     "src/compiler/liveness-analyzer.h",
     "src/compiler/load-elimination.cc",
@@ skipping 1013 matching lines @@
         "src/d8-debug.cc",
         "src/d8-debug.h",
         "$target_gen_dir/d8-js.cc",
       ]
     }
     if (v8_enable_i18n_support) {
       deps += [ "//third_party/icu" ]
     }
   }
 }