[turbofan] Don't spread global flag checks all over the compiler code.

src/compiler/source-position.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_SOURCE_POSITION_H_
 #define V8_COMPILER_SOURCE_POSITION_H_
 
 #include "src/assembler.h"
 #include "src/compiler/node-aux-data.h"
 
@@ skipping 27 matching lines @@
   return lhs.raw() == rhs.raw();
 }
 
 inline bool operator!=(const SourcePosition& lhs, const SourcePosition& rhs) {
   return !(lhs == rhs);
 }
 
 
 class SourcePositionTable final {
  public:
-  class Scope {
+  class Scope final {
    public:
     Scope(SourcePositionTable* source_positions, SourcePosition position)
         : source_positions_(source_positions),
           prev_position_(source_positions->current_position_) {
       Init(position);
     }
     Scope(SourcePositionTable* source_positions, Node* node)
         : source_positions_(source_positions),
           prev_position_(source_positions->current_position_) {
       Init(source_positions_->GetSourcePosition(node));
     }
     ~Scope() { source_positions_->current_position_ = prev_position_; }
 
    private:
     void Init(SourcePosition position) {
       if (!position.IsUnknown() || prev_position_.IsInvalid()) {
         source_positions_->current_position_ = position;
       }
     }
 
-    SourcePositionTable* source_positions_;
-    SourcePosition prev_position_;
+    SourcePositionTable* const source_positions_;
+    SourcePosition const prev_position_;
     DISALLOW_COPY_AND_ASSIGN(Scope);
   };
 
   explicit SourcePositionTable(Graph* graph);
   ~SourcePositionTable() {
-    if (decorator_ != NULL) RemoveDecorator();
+    if (decorator_) RemoveDecorator();
   }
 
   void AddDecorator();
   void RemoveDecorator();
 
   SourcePosition GetSourcePosition(Node* node) const;
 
   void Print(std::ostream& os) const;
 
  private:
   class Decorator;
 
-  Graph* graph_;
+  Graph* const graph_;
   Decorator* decorator_;
   SourcePosition current_position_;
   NodeAuxData<SourcePosition> table_;
 
   DISALLOW_COPY_AND_ASSIGN(SourcePositionTable);
 };
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8
 
-#endif
+#endif  // V8_COMPILER_SOURCE_POSITION_H_