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

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
diff --git a/src/ia32/macro-assembler-ia32.h b/src/ia32/macro-assembler-ia32.h
index e62c7d8b4d23d6013b0cddda89db67be31c96773..888fe69b927b9250fae91d9e2a14e6e1f71b920f 100644
--- a/src/ia32/macro-assembler-ia32.h
+++ b/src/ia32/macro-assembler-ia32.h
@@ -718,6 +718,10 @@ 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