RFR: 8353115: GenShen: mixed evacuation candidate regions need accurate live_data [v4]

[Kelvin Nilsen]

> The existing implementation of get_live_data_bytes() and git_live_data_words() does not always behave as might be expected.  In particular, the value returned ignores any allocations that occur subsequent to the most recent mark effort that identified live data within the region.  This is typically ok for young regions that are going to be added or not to the collection set during final-mark safepoint.
> 
> However, old-gen regions that are placed into the set of candidates for mixed evacuation are more complicated.  In particular, by the time the old-gen region is added to a mixed evacuation, its live data may be much larger than at the time concurrent old marking ended.
> 
> This PR provides comments to clarify the shortcomings of the existing functions, and adds new functions that provide more accurate accountings of live data for mixed-evacuation candidate regions.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Fix garbage_before_padded_for_promote()

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24319/files
  - new: https://git.openjdk.org/jdk/pull/24319/files/8ff388d1..8e820f29

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24319&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24319&range=02-03

  Stats: 6 lines in 1 file changed: 3 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24319.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24319/head:pull/24319

PR: https://git.openjdk.org/jdk/pull/24319