RFR: Hook up Shenandoah barrier expand to ShBSC2::expand_macro_nodes()

[Roland Westrelin]

> Would you try to upstream the non-Shenandoah changes asap, and then push
> the Shenandoah parts to shenandoah/jdk ?

Yes. But first need to make sure I don't somehow break ZGC.

Roland.