RFR: 8332913: GenShen: Restore shared update refs iterator

[William Kemper]

Turns out, this iterator is shared between concurrent and degenerated GC cycles. It should not have been made into a local variable.

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Commit messages:
 - Restore shared update refs iterator

Changes: https://git.openjdk.org/shenandoah/pull/439/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=439&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332913
  Stats: 23 lines in 4 files changed: 10 ins; 5 del; 8 mod
  Patch: https://git.openjdk.org/shenandoah/pull/439.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/439/head:pull/439

PR: https://git.openjdk.org/shenandoah/pull/439