RFR: Improve humongous remset scan

[Kelvin Nilsen]

These changes allow humongous objects residing within the remembered set to be scanned by muitiple concurrent GC worker threads, allowing more efficient parallelization of the remembered set scanning effort.

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Commit messages:
 - Merge remote-tracking branch 'GitFarmBranch/optimize-humongous-remset-scan' into improve-humongous-remset-scan
 - Fix handling of humongous continuation regions
 - Merge branch 'optimize-humongous-remset-scan' of ssh://git.amazon.com/pkg/OpenJDKTipSrc into optimize-humongous-remset-scan
 - Start young collect instead of old if mixed evacuations are pending
 - Filter out invalid pointers picked up by old gen SATB
 - Fix typo in assertion
 - Add missing semicolon
 - Another fix to round-up calculation
 - Fix error in round-up calculation
 - Require humongous slice to be multiple of card size
 - ... and 14 more: https://git.openjdk.org/shenandoah/compare/a8676f6d...3b558e0a

Changes: https://git.openjdk.org/shenandoah/pull/144/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=144&range=00
  Stats: 70 lines in 3 files changed: 69 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/144.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/144/head:pull/144

PR: https://git.openjdk.org/shenandoah/pull/144