Introduce FieldIndex to unify and abstract property/field offset

src/ic.h

Index: src/ic.h
diff --git a/src/ic.h b/src/ic.h
index 5b4100abaf21cfa2cc14926d5c30dca18527c005..f3e2418ee61d2a0ce5286900be19c3dc88b61640 100644
--- a/src/ic.h
+++ b/src/ic.h
@@ -481,10 +481,7 @@ class LoadIC: public IC {
     return pre_monomorphic_stub(isolate(), extra_ic_state());
   }
 
-  Handle<Code> SimpleFieldLoad(int offset,
-                               bool inobject = true,
-                               Representation representation =
-                                    Representation::Tagged());
+  Handle<Code> SimpleFieldLoad(FieldIndex index);
 
   static void Clear(Isolate* isolate,
                     Address address,