RFR: 8290019: Refactor HeapRegion::oops_on_memregion_iterate()

[Thomas Schatzl]

Hi all,

  please review this potential refactoring for `HeapRegion::oops_on_memregion_iterate()`; it's a bit subjective imo if the code is better now, so please feel free to say so and I'll retract.

Testing: tier1-3

Thanks,
  Thomas

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Commit messages:
 - Initail version

Changes: https://git.openjdk.org/jdk/pull/9430/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9430&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290019
  Stats: 45 lines in 2 files changed: 18 ins; 18 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/9430.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9430/head:pull/9430

PR: https://git.openjdk.org/jdk/pull/9430