RFR: 8338440: Parallel: Improve fragmentation mitigation in Full GC [v2]

[Albert Mingkun Yang]

> Extend `SplitInfo` to support more fine-grained splitting to mitigate the fragmentation issue during full GC. Added comments and diagrams in the process.
> 
> For easier review, it's best to start with `SplitInfo` and then proceed to see how it is constructed in `summarize_split_space` and consumed in `first_src_addr`. The accompanying diagrams should help create a clear mental image.
> 
> With this patch, the exec time of `runtime/ClassInitErrors/TestOutOfMemoryDuringInit.java` using Parallel drops from ~30s to ~8s, the same as other GCs, and gc-log shows similar number of GC cycles as well.
> 
> Test: tier1-8, systemgc micro bm, CacheStress, dacapo, specjbb2005, specjvm2008

Albert Mingkun Yang has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - review
 - Merge branch 'master' into pgc-split-region
 - pgc-split-region

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20590/files
  - new: https://git.openjdk.org/jdk/pull/20590/files/38cc30c2..a4665329

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20590&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20590&range=00-01

  Stats: 15756 lines in 381 files changed: 10446 ins; 3328 del; 1982 mod
  Patch: https://git.openjdk.org/jdk/pull/20590.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20590/head:pull/20590

PR: https://git.openjdk.org/jdk/pull/20590