RFR: 8273476: G1: refine G1CollectedHeap::par_iterate_regions_array_part_from

[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.

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Commit messages:
 - refine par_iterate_regions_array_part_from

Changes: https://git.openjdk.java.net/jdk/pull/5414/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5414&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273476
  Stats: 10 lines in 4 files changed: 5 ins; 3 del; 2 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