RFR: 8329603: G1: Merge G1BlockOffsetTablePart into G1BlockOffsetTable

[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

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Commit messages:
 - JDK-8329603

Changes: https://git.openjdk.org/jdk/pull/18634/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18634&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329603
  Stats: 165 lines in 10 files changed: 31 ins; 56 del; 78 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