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

[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.

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Commit messages:
 - do not count promoted-in-place live data toward TAMS old live data

Changes: https://git.openjdk.org/jdk/pull/28837/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28837&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373720
  Stats: 19 lines in 4 files changed: 17 ins; 1 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