RFR: 8329789: GenShen: Over assertive assert when scanning remembered set

[William Kemper]

When scanning the remembered set for update-refs, it is possible for the scanner to encounter a card that was dirtied by a mutator during the scan. This is by design.

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Commit messages:
 - Soften assertion when using the write card table

Changes: https://git.openjdk.org/shenandoah/pull/418/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=418&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329789
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/418.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/418/head:pull/418

PR: https://git.openjdk.org/shenandoah/pull/418