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

[Albert Mingkun Yang]

On Fri, 16 Sep 2022 09:30:14 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> 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

Thanks for the review.

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