RFR: 8234974: Shenandoah: Do concurrent roots even when no evacuation is necessary

[Roman Kennke]

Shenandoah can short-cut a cycle when the collection set remains empty,
and doesn't dive into concurrent evacuation and updating refs phases
then. However, this currently also precludes concurrent roots processing
and concurrent class unloading. This is only a minor nuisance now
(effectively skipping class unloading for short-cut-cycles), but amounts
to a real bug when we're going to do weak-roots-cleaning concurrently too.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8234974
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8234974/webrev.00/

Testing: hotspot_gc_shenandoah

Can I please get a review?

Roman