X87: Move Maps' back pointers from "transitions" to "constructor" field.

src/x87/macro-assembler-x87.h

Index: src/x87/macro-assembler-x87.h
diff --git a/src/x87/macro-assembler-x87.h b/src/x87/macro-assembler-x87.h
index c25203f2d1d03e6ce2c959ccfdb07dbf10b67fd2..9d9556c5f2dabc8e64be1f61db1edc3069b5b8bd 100644
--- a/src/x87/macro-assembler-x87.h
+++ b/src/x87/macro-assembler-x87.h
@@ -683,6 +683,10 @@ class MacroAssembler: public Assembler {
   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