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

src/arm64/full-codegen-arm64.cc

Index: src/arm64/full-codegen-arm64.cc
diff --git a/src/arm64/full-codegen-arm64.cc b/src/arm64/full-codegen-arm64.cc
index 265f75f97c4fd994e0f00373c5d21d89d9417105..d2e48b23121e65a407f46f2efa4e41af4cf081b1 100644
--- a/src/arm64/full-codegen-arm64.cc
+++ b/src/arm64/full-codegen-arm64.cc
@@ -3484,9 +3484,10 @@ 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(x12, FieldMemOperand(x10, Map::kConstructorOffset));
-  __ JumpIfNotObjectType(x12, x13, x14, JS_FUNCTION_TYPE,
-                         &non_function_constructor);
+  Register instance_type = x14;
+  __ GetMapConstructor(x12, x10, x13, instance_type);
+  __ Cmp(instance_type, JS_FUNCTION_TYPE);
+  __ B(ne, &non_function_constructor);
 
   // x12 now contains the constructor function. Grab the
   // instance class name from there.