RFR: 8373720: GenShen: Count live-at-old mark using Snapshot at Beginning [v3]

[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 incrementally with one additional commit since the last revision:

  fix up comments

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28837/files
  - new: https://git.openjdk.org/jdk/pull/28837/files/f9ee6646..5a53f72c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28837&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28837&range=01-02

  Stats: 5 lines in 1 file changed: 3 ins; 2 del; 0 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