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

src/compiler/frame.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_FRAME_H_
 #define V8_COMPILER_FRAME_H_
 
 #include "src/bit-vector.h"
 #include "src/frames.h"
 
@@ skipping 103 matching lines @@
   }
   inline int GetSpillSlotCount() const { return spill_slot_count_; }
 
   inline void SetElidedFrameSizeInSlots(int slots) {
     DCHECK_EQ(0, callee_saved_slot_count_);
     DCHECK_EQ(0, spill_slot_count_);
     frame_slot_count_ = slots;
   }
 
   void SetAllocatedRegisters(BitVector* regs) {
-    DCHECK(allocated_registers_ == NULL);
+    DCHECK(allocated_registers_ == nullptr);
     allocated_registers_ = regs;
   }
 
   void SetAllocatedDoubleRegisters(BitVector* regs) {
-    DCHECK(allocated_double_registers_ == NULL);
+    DCHECK(allocated_double_registers_ == nullptr);
     allocated_double_registers_ = regs;
   }
 
   bool DidAllocateDoubleRegisters() const {
     return !allocated_double_registers_->IsEmpty();
   }
 
   int AlignSavedCalleeRegisterSlots() {
     DCHECK_EQ(0, callee_saved_slot_count_);
     needs_frame_ = true;
@@ skipping 108 matching lines @@
  private:
   Frame* const frame_;
   bool access_frame_with_fp_;
   int sp_delta_;
 };
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COMPILER_FRAME_H_