RFR: 8273476: G1: refine G1CollectedHeap::par_iterate_regions_array_part_from [v2]

[Hamlin Li]

> G1CollectedHeap::par_iterate_regions_array_part_from is moved from G1CollectionSet::iterate_part_from in JDK-8254167.
> The parameters and logic of this method could be refined a bit to make it more straight.

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  refine comments and code

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5414/files
  - new: https://git.openjdk.java.net/jdk/pull/5414/files/6292fd20..78da4ce7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5414&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5414&range=00-01

  Stats: 23 lines in 4 files changed: 0 ins; 0 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5414.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5414/head:pull/5414

PR: https://git.openjdk.java.net/jdk/pull/5414