RFR: keep read barrier on final field access

[Roland Westrelin]

> Do we want to wire it up with TrustFinalNonStaticFields? I.e. if 
> TrustFinalNonStaticFields is set, also set our own flag?

That seems risky to me and I think it would be best that
ShenandoahOptimizeInstanceFinals be enabled only if explicitly
requested.

Here is a new webrev:

http://cr.openjdk.java.net/~roland/shenandoah/rbonfinalinstancefield/webrev.01/

Roland.