RFR (S) 8242273: Shenandoah: accept either SATB or IU barriers, but not both

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8242273

The follow-up for JDK-8242054, we need to make sure users cannot enable both SATB and IU barriers
together. Currently Shenandoah blindly accepts +ShenandoahSATBBarrier
+ShenandoahStoreValEnqueueBarrier, which might lead to surprising behaviors.

Since "mode" is what defines the barriers, I moved all relevant barrier checking code there.

Fix:
  https://cr.openjdk.java.net/~shade/8242273/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey