RFR: 8332913: GenShen: Restore shared update refs iterator [v2]

[William Kemper]

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

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Restore card stats enable check

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/439/files
  - new: https://git.openjdk.org/shenandoah/pull/439/files/6fdc1691..643edc23

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=439&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=439&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 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