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

src/compiler/graph-visualizer.h

 // Copyright 2013 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_GRAPH_VISUALIZER_H_
 #define V8_COMPILER_GRAPH_VISUALIZER_H_
 
 #include <stdio.h>
 #include <iosfwd>
 
@@ skipping 30 matching lines @@
 
 
 struct AsC1VCompilation {
   explicit AsC1VCompilation(const CompilationInfo* info) : info_(info) {}
   const CompilationInfo* info_;
 };
 
 
 struct AsC1V {
   AsC1V(const char* phase, const Schedule* schedule,
-        const SourcePositionTable* positions = NULL,
-        const InstructionSequence* instructions = NULL)
+        const SourcePositionTable* positions = nullptr,
+        const InstructionSequence* instructions = nullptr)
       : schedule_(schedule),
         instructions_(instructions),
         positions_(positions),
         phase_(phase) {}
   const Schedule* schedule_;
   const InstructionSequence* instructions_;
   const SourcePositionTable* positions_;
   const char* phase_;
 };
 
 struct AsC1VRegisterAllocationData {
   explicit AsC1VRegisterAllocationData(
       const char* phase, const RegisterAllocationData* data = nullptr)
       : phase_(phase), data_(data) {}
   const char* phase_;
   const RegisterAllocationData* data_;
 };
 
 std::ostream& operator<<(std::ostream& os, const AsC1VCompilation& ac);
 std::ostream& operator<<(std::ostream& os, const AsC1V& ac);
 std::ostream& operator<<(std::ostream& os,
                          const AsC1VRegisterAllocationData& ac);
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_GRAPH_VISUALIZER_H_