RFR: 8264908: Investigate adding BOT range check in G1BlockOffsetTablePart::block_at_or_preceding

[Ivan Walulya]

Hi all,

Please review this cleanup change to add an assert to G1BlockOffsetTablePart::block_at_or_preceding. Additionally, remove code that attempts to read beyond the _next_offset_index threshold. 

Testing: Tier 1-3

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Commit messages:
 - refactor G1BlockOffsetTablePart::block_at_or_preceding to add assert precondition

Changes: https://git.openjdk.java.net/jdk/pull/4775/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4775&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264908
  Stats: 60 lines in 2 files changed: 0 ins; 56 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4775.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4775/head:pull/4775

PR: https://git.openjdk.java.net/jdk/pull/4775