RFR: 8332851: Shenandoah: Update refs iterator does not need to be a member of ShenandoahHeap

[William Kemper]

The iterator is used outside of Shenandoah heap to support an assert, and must be 'reset' for correctness. These iterators are typically used as a local variable, and we can do that with this iterator as well to simplify the code.

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Commit messages:
 - Use local variable for update refs iterator

Changes: https://git.openjdk.org/jdk/pull/19376/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19376&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332851
  Stats: 9 lines in 3 files changed: 1 ins; 5 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19376.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19376/head:pull/19376

PR: https://git.openjdk.org/jdk/pull/19376