[turbofan] Introduce separate SelectLowering reducer.

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.
 
 # TODO(jochen): These will need to be user-settable to support standalone V8
 # builds.
 v8_compress_startup_data = "off"
 v8_deprecation_warnings = false
 v8_enable_disassembler = false
 v8_enable_gdbjit = false
@@ skipping 558 matching lines @@
     "src/compiler/pipeline-statistics.h",
     "src/compiler/raw-machine-assembler.cc",
     "src/compiler/raw-machine-assembler.h",
     "src/compiler/register-allocator.cc",
     "src/compiler/register-allocator.h",
     "src/compiler/representation-change.h",
     "src/compiler/schedule.cc",
     "src/compiler/schedule.h",
     "src/compiler/scheduler.cc",
     "src/compiler/scheduler.h",
+    "src/compiler/select-lowering.cc",
+    "src/compiler/select-lowering.h",
     "src/compiler/simplified-lowering.cc",
     "src/compiler/simplified-lowering.h",
     "src/compiler/simplified-operator-reducer.cc",
     "src/compiler/simplified-operator-reducer.h",
     "src/compiler/simplified-operator.cc",
     "src/compiler/simplified-operator.h",
     "src/compiler/source-position.cc",
     "src/compiler/source-position.h",
     "src/compiler/typer.cc",
     "src/compiler/typer.h",
@@ skipping 857 matching lines @@
     deps = [
       ":v8_base",
       ":v8_nosnapshot",
     ]
   }
 
   direct_dependent_configs = [ ":external_config" ]
 }
 
 }