RFR: keep read barrier on final field access

[Roland Westrelin]

> Like this? Basically your original version, simplified and indented:
>
>   if ((ShenandoahOptimizeStaticFinals   && field->is_final() &&  field->is_static()) ||
>       (ShenandoahOptimizeInstanceFinals && field->is_final() && !field->is_static()) ||
>       (ShenandoahOptimizeStableFinals   && field->is_stable())) {
>     // Skip the barrier for special fields
>   } else {
>     obj = shenandoah_read_barrier(obj);
>   }

Fine with me. Roman, can I push that version?

Roland.