Helpers for getClass and identityHashCode

The map in GenJSCode is related but different. It maps method symbols to helpers. Method symbols are used to create the dce info, and `getClass` is explicitly excluded from this treatment [there](https://github.com/scala-js/scala-js/pull/1044/files#diff-7787dd6a2f43dd4a8b4af6391a056537R1557).
Here, this is the map that is used to reconstruct dce info from the trees in the optimized method. So here we just ignore `objectGetClass`, which means that it never marks anything reachable.