RFR: 8271930: Simplify end_card calculation in G1BlockOffsetTablePart::verify [v2]

[Albert Mingkun Yang]

> Simplify `end_card` calculation and some general cleanup.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5015/files
  - new: https://git.openjdk.java.net/jdk/pull/5015/files/82b0533c..e9157aaa

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5015&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5015&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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