RFR: 8373720: GenShen: Count live-at-old mark using Snapshot at Beginning [v4]

[Kelvin Nilsen]

> Live memory in old is measured as of the start of old-generation concurrent marking.
> 
> Memory promoted during concurrent old marking (memory above TAMS for the old-generation heap regions) and memory regions promoted in place during concurrent old marking are excluded from the total live data at start of old marking.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge remote-tracking branch 'jdk/master' into fix-live-data-at-old-mark-gh
 - fix up comments
 - Merge remote-tracking branch 'jdk/master' into fix-live-data-at-old-mark
 - do not count promoted-in-place live data toward TAMS old live data

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Changes: https://git.openjdk.org/jdk/pull/28837/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28837&range=03
  Stats: 19 lines in 4 files changed: 16 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28837.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28837/head:pull/28837

PR: https://git.openjdk.org/jdk/pull/28837