RFR(sh/jdk8): Bulk backports 2020-12-08

[Roman Kennke]

This backports the following fixes to shenandoah/jdk8:

8244510: Shenandoah: invert SHC2Support::is_in_cset condition:
https://bugs.openjdk.java.net/browse/JDK-8244510

8244721: CTW: C2 (Shenandoah) compilation fails with "unexpected 
infinite loop graph shape":
https://bugs.openjdk.java.net/browse/JDK-8244721

8251451: Shenandoah: Remark ObjectSynchronizer roots with I-U:
https://bugs.openjdk.java.net/browse/JDK-8251451

8253224: Shenandoah: ShenandoahStrDedupQueue destructor calls virtual 
num_queues():
https://bugs.openjdk.java.net/browse/JDK-8253224

All patches have been applied fairly straightforward:
http://cr.openjdk.java.net/~rkennke/backport-jdk8-2020-12-08/webrev.00/

Testing: hotspot_gc_shenandoah

ok?

Roman