RFR: keep read barrier on final field access

[Roland Westrelin]

> I wonder if that means we cannot figure out early in GraphKit if the field access would be "stable"
> or not, for fields that are marked for trustness in ciField::trust_final_non_static_fields. I do
> think we need to walk through all the different trustness levels, including @Stable, +TFNSF,
> ciField:tfnsf, etc, and capture what they have in Shenandoah.

Do we have any evidence that classes for which
ciField::trust_final_non_static_fields() returns true today would work
ok with no read barriers on instance final fields?

What ciField::trust_final_non_static_fields() says AFAIU is that it's
guaranteed the final field is not updated by reflection etc. outside the
constructor and that's it.

Given how tricky it is to track down a missing read barrier, I think it
would be safer to be conservative here.

Roland.