RFR: 8293920: G1: Add index based heap region iteration

Fri Sep 16 09:38:38 UTC 2022

Simple change of introducing a new `iterate` method. It's dead code for now, and will be used soon in [JDK-8293210](https://bugs.openjdk.org/browse/JDK-8293210).

Test: hotspot_gc

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Commit messages:
 - iterator

Changes: https://git.openjdk.org/jdk/pull/10301/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293920
  Stats: 40 lines in 5 files changed: 40 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10301.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10301/head:pull/10301

PR: https://git.openjdk.org/jdk/pull/10301