[turbofan] Build s/NULL/nullptr/g and CHECK(x != nullptr) to CHECK_NOT_NULL(x).

src/compiler/zone-pool.h

 // 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.
 
 #ifndef V8_COMPILER_ZONE_POOL_H_
 #define V8_COMPILER_ZONE_POOL_H_
 
 #include <map>
 #include <set>
 #include <vector>
 
 #include "src/zone.h"
 
 namespace v8 {
 namespace internal {
 namespace compiler {
 
 class ZonePool final {
  public:
   class Scope final {
    public:
-    explicit Scope(ZonePool* zone_pool) : zone_pool_(zone_pool), zone_(NULL) {}
+    explicit Scope(ZonePool* zone_pool)
+        : zone_pool_(zone_pool), zone_(nullptr) {}
     ~Scope() { Destroy(); }
 
     Zone* zone() {
-      if (zone_ == NULL) zone_ = zone_pool_->NewEmptyZone();
+      if (zone_ == nullptr) zone_ = zone_pool_->NewEmptyZone();
       return zone_;
     }
     void Destroy() {
-      if (zone_ != NULL) zone_pool_->ReturnZone(zone_);
-      zone_ = NULL;
+      if (zone_ != nullptr) zone_pool_->ReturnZone(zone_);
+      zone_ = nullptr;
     }
 
    private:
     ZonePool* const zone_pool_;
     Zone* zone_;
     DISALLOW_COPY_AND_ASSIGN(Scope);
   };
 
   class StatsScope final {
    public:
@@ skipping 41 matching lines @@
   size_t total_deleted_bytes_;
 
   DISALLOW_COPY_AND_ASSIGN(ZonePool);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif