[11] RFR: 2020-05-19, Bulk backports to sh/jdk11

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk11-20200519/webrev.01/

This backports C2 changes. 8244509 was applied by accepting the incoming code shape wholesale.
8244663 and 8244721 regression tests were verified to fail before and passing after. I skipped
8245083 (null-ptr stuff), because it was problematic before, and needs more time to soak.

Changes:
 [backport] 8244663: Shenandoah: C2 assertion fails in Matcher::collect_null_checks
 [backport] 8244721: CTW: C2 (Shenandoah) compilation fails with "unexpected infinite loop graph
 [backport] 8244509: Shenandoah: refactor ShenandoahBarrierC2Support::test_* methods
 [backport] 8244667: Shenandoah: SBC2Support::test_gc_state takes loop for wrong control
 [backport] 8244510: Shenandoah: invert SHC2Support::is_in_cset condition

Testing: hotspot_gc_shenandoah {fastdebug,release}, tier{1,2} with Shenandoah (in progress)

-- 
Thanks,
-Aleksey