Make it clear that HInstruction's position is a write-once variable.

src/hydrogen-instructions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 839 matching lines @@
   HInstruction* previous() const { return previous_; }
 
   virtual void PrintTo(StringStream* stream);
   virtual void PrintDataTo(StringStream* stream) { }
 
   bool IsLinked() const { return block() != NULL; }
   void Unlink();
   void InsertBefore(HInstruction* next);
   void InsertAfter(HInstruction* previous);
 
+  // The position is a write-once variable.
   int position() const { return position_; }
   bool has_position() const { return position_ != RelocInfo::kNoPosition; }
-  void set_position(int position) { position_ = position; }
+  void set_position(int position) {
+    ASSERT(!has_position());
+    ASSERT(position != RelocInfo::kNoPosition);
+    position_ = position;
+  }
 
   bool CanTruncateToInt32() const { return CheckFlag(kTruncatingToInt32); }
 
   virtual LInstruction* CompileToLithium(LChunkBuilder* builder) = 0;
 
 #ifdef DEBUG
   virtual void Verify();
 #endif
 
   virtual bool IsCall() { return false; }
@@ skipping 4272 matching lines @@
   DECLARE_CONCRETE_INSTRUCTION(LoadFieldByIndex);
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_