RFR (XS): -ShenandoahSATBBarrier disables matrix updates

[Aleksey Shipilev]

Hi,

While studying the generated code, I realized that -ShenandoahSATBBuffer
disables matrix updates too. Oops. This does not affect our performance runs
just yet, because we enable SATB barriers before enabling matrix barriers. We
should fix this oddity nevertheless:
 http://cr.openjdk.java.net/~shade/shenandoah/satb-barriers-disable-2/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing assembly

Thanks,
-Aleksey