[8u] 2020-08-12, Bulk backports to sh/jdk8

[Aleksey Shipilev]

https://cr.openjdk.java.net/~shade/shenandoah/backports/jdk8u-20200812/webrev.01/

This backports some C2 refactoring and cset encoding changes:
 [backport] 8245464: Shenandoah: allocate collection set bitmap at lower addresses
 [backport] 8245773: Shenandoah: Windows assertion failure after JDK-8245464
 [backport] 8244509: Shenandoah: refactor ShenandoahBarrierC2Support::test_* methods
 [backport] 8244667: Shenandoah: SBC2Support::test_gc_state takes loop for wrong control
 [backport] 8245465: Shenandoah: test_in_cset can use more efficient encoding

Testing: hotspot_gc_shenadoah {fastdebug,release}; eyeballing generated code; adhoc benchmarks

-- 
Thanks,
-Aleksey