RFR: 8278139: G1: Refactor G1BlockOffsetTablePart::block_at_or_preceding

[Albert Mingkun Yang]

Simple change of removing some redundant calculation.

Test: hotspot_gc

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Commit messages:
 - block_at_or_preceding

Changes: https://git.openjdk.java.net/jdk/pull/6666/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6666&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278139
  Stats: 6 lines in 1 file changed: 1 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6666.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6666/head:pull/6666

PR: https://git.openjdk.java.net/jdk/pull/6666