Move Maps' back pointers from "transitions" to "constructor" field

src/x64/macro-assembler-x64.h

 // Copyright 2012 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_X64_MACRO_ASSEMBLER_X64_H_
 #define V8_X64_MACRO_ASSEMBLER_X64_H_
 
 #include "src/assembler.h"
 #include "src/bailout-reason.h"
 #include "src/frames.h"
@@ skipping 1228 matching lines @@
   void NegativeZeroTest(CodeGenerator* cgen,
                         Register result,
                         Register op,
                         JumpTarget* then_target);
 
   // Check if result is zero and any of op1 and op2 are negative.
   // Register scratch is destroyed, and it must be different from op2.
   void NegativeZeroTest(Register result, Register op1, Register op2,
                         Register scratch, Label* then_label);
 
+  // Machine code version of Map::GetConstructor().
+  // |temp| holds |result|'s map when done.
+  void GetMapConstructor(Register result, Register map, Register temp);
+
   // Try to get function prototype of a function and puts the value in
   // the result register. Checks that the function really is a
   // function and jumps to the miss label if the fast checks fail. The
   // function register will be untouched; the other register may be
   // clobbered.
   void TryGetFunctionPrototype(Register function,
                                Register result,
                                Label* miss,
                                bool miss_on_bound_function = false);
 
@@ skipping 375 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_