RFR: 8271930: Simplify end_card calculation in G1BlockOffsetTablePart::verify

[Albert Mingkun Yang]

Simplify `end_card` calculation and some general cleanup.

Test: hotspot_gc

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Commit messages:
 - bot-old

Changes: https://git.openjdk.java.net/jdk/pull/5015/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5015&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271930
  Stats: 18 lines in 3 files changed: 1 ins; 12 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5015.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5015/head:pull/5015

PR: https://git.openjdk.java.net/jdk/pull/5015