Integrated: 8338717: GenShen: Fix up some assertions

[William Kemper]

Clean.

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Commit messages:
 - Backport d7e22354f30a1260fc55006cafe8fd80bdad9e04

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/92/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=92&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338717
  Stats: 74 lines in 9 files changed: 49 ins; 8 del; 17 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/92.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/92/head:pull/92

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/92