RFR: keep read barrier on final field access

[Roland Westrelin]

As discussed in [1]:

http://cr.openjdk.java.net/~roland/shenandoah/rbonfinalinstancefield/webrev.00/

Roland.

[1] http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-October/004063.html