RFR: 8287555: Tighten G1 G1BlockOffsetTable::block_start() code

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that specializes `G1BlockOffsetTablePart::block_size` with a variant used for card aligned addresses including some cleanup of comments? This saves some unnecessary lookup of the block size of the object found as for refinement the start address is always aligned.

Testing: tier1, gha

Thanks,
  Thomas

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Commit messages:
 - Fix whitespace
 - Cleanup
 - 8287555: Tighten G1BlockOffsetTable::block_start() code

Changes: https://git.openjdk.org/jdk/pull/9059/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9059&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287555
  Stats: 41 lines in 5 files changed: 28 ins; 2 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/9059.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9059/head:pull/9059

PR: https://git.openjdk.org/jdk/pull/9059