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

src/x64/lithium-codegen-x64.cc

Index: src/x64/lithium-codegen-x64.cc
diff --git a/src/x64/lithium-codegen-x64.cc b/src/x64/lithium-codegen-x64.cc
index 0f0d136b91d78abeef55792e200c6461fbfe7820..0345ca429571642db0c62b6cbcd33e2b67f27aa7 100644
--- a/src/x64/lithium-codegen-x64.cc
+++ b/src/x64/lithium-codegen-x64.cc
@@ -2615,10 +2615,10 @@ void LCodeGen::EmitClassOfTest(Label* is_true,
 
   // Now we are in the FIRST-LAST_NONCALLABLE_SPEC_OBJECT_TYPE range.
   // Check if the constructor in the map is a function.
-  __ movp(temp, FieldOperand(temp, Map::kConstructorOffset));
+  __ GetMapConstructor(temp, temp, kScratchRegister);
 
   // Objects with a non-function constructor have class 'Object'.
-  __ CmpObjectType(temp, JS_FUNCTION_TYPE, kScratchRegister);
+  __ CmpInstanceType(kScratchRegister, JS_FUNCTION_TYPE);
   if (String::Equals(class_name, isolate()->factory()->Object_string())) {
     __ j(not_equal, is_true);
   } else {