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

[William Kemper]

Clean backport, assert only and assert only applies to non-product code.

-------------

Commit messages:
 - Backport 69281629a4a537b988f5dfd3a93733b3ddccd109

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/34/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329789
  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/34/head:pull/34

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/34