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

src/arm64/macro-assembler-arm64.h

Index: src/arm64/macro-assembler-arm64.h
diff --git a/src/arm64/macro-assembler-arm64.h b/src/arm64/macro-assembler-arm64.h
index 3a0df0b04926bce5e68bffba7ddb9b0ff40902ff..347e0654be06b98a58a4241cbaee9dbc116dc7e5 100644
--- a/src/arm64/macro-assembler-arm64.h
+++ b/src/arm64/macro-assembler-arm64.h
@@ -1378,6 +1378,11 @@ class MacroAssembler : public Assembler {
     kDontMissOnBoundFunction
   };
 
+  // Machine code version of Map::GetConstructor().
+  // |temp| holds |result|'s map when done, and |temp2| its instance type.
+  void GetMapConstructor(Register result, Register map, Register temp,
+                         Register temp2);
+
   void TryGetFunctionPrototype(Register function,
                                Register result,
                                Register scratch,