RFR: keep read barrier on final field access

[Roland Westrelin]

Roman,

Is it ok with you if I go with this suggestion?

> if ((ShenandoahOptimiseStaticFinals && field->is_static()) ||
>     (ShenandoahOptimizeInstanceFinals && field->is_final()) ||
>     (ShenandoahOptimizeStableFinals && field->is_stable())

Roland.