RFR: 8289538: Make G1BlockOffsetTablePart unaware of block sizes

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes `G1BlockOffsetTablePart` unaware of block sizes: as discussed in PR#9059/[JDK-8287555](https://bugs.openjdk.org/browse/JDK-8287555) this is a good idea to make `G1BlockOffsetTablePart` just for "dumb" retrieval of the BOT entries, which makes the code simpler, avoiding some calling back and forth between `HeapRegion` and `G1BlockOffsetTablePart`.

There is still the wart that `G1BlockOffsetTablePart::verify()` needs `block_size`, but since it's just verification I think this is fine. An alternative suggested by @albertnetymk is to remove that method completely.

Testing: local compilation, gha, some benchmark checking that card scans/ms does not degrade

Thanks,
  Thomas

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Commit messages:
 - initial version

Changes: https://git.openjdk.org/jdk/pull/9395/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9395&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289538
  Stats: 165 lines in 6 files changed: 70 ins; 80 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/9395.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9395/head:pull/9395

PR: https://git.openjdk.org/jdk/pull/9395