RFR: 8329603: G1: Merge G1BlockOffsetTablePart into G1BlockOffsetTable [v3]

[Guoxiong Li]

> Hi all,
> 
> This patch merges `G1BlockOffsetTablePart` into `G1BlockOffsetTable`. The previous fields `_reserved` and `_offset_base` of `G1BlockOffsetTable` are marked as `static` so that they can be shared by BOTs of all the heap regions.
> 
> The tests `make test-tier1_gc` passed locally. Thanks for taking the time to review.
> 
> Best Regards,
> -- Guoxiong

Guoxiong Li has updated the pull request incrementally with two additional commits since the last revision:

 - Remove unnecessary comments.
 - Fix indentation issue.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18634/files
  - new: https://git.openjdk.org/jdk/pull/18634/files/ccafb2f7..a8a121bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18634&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18634&range=01-02

  Stats: 5 lines in 2 files changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18634.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18634/head:pull/18634

PR: https://git.openjdk.org/jdk/pull/18634