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

src/arm/full-codegen-arm.cc

Index: src/arm/full-codegen-arm.cc
diff --git a/src/arm/full-codegen-arm.cc b/src/arm/full-codegen-arm.cc
index 15958ccf54561149055489f32188e1530473fb81..f433990d23a9b41c171c1fc9e987dbe004730ed7 100644
--- a/src/arm/full-codegen-arm.cc
+++ b/src/arm/full-codegen-arm.cc
@@ -3777,8 +3777,9 @@ void FullCodeGenerator::EmitClassOf(CallRuntime* expr) {
   STATIC_ASSERT(LAST_NONCALLABLE_SPEC_OBJECT_TYPE == LAST_TYPE - 1);
 
   // Check if the constructor in the map is a JS function.
-  __ ldr(r0, FieldMemOperand(r0, Map::kConstructorOffset));
-  __ CompareObjectType(r0, r1, r1, JS_FUNCTION_TYPE);
+  Register instance_type = r2;
+  __ GetMapConstructor(r0, r0, r1, instance_type);
+  __ cmp(instance_type, Operand(JS_FUNCTION_TYPE));
   __ b(ne, &non_function_constructor);
 
   // r0 now contains the constructor function. Grab the