RFR: 8360288: Shenandoah crash at size_given_klass in op_degenerated

[William Kemper]

Clean backport. Fixes a crash in Shenandoah (ILW = P3). Only touches Shenandoah code.

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Commit messages:
 - Backport 3b44d7bfa4d78e3ec715fce1863e052852f33180

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/206/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=206&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360288
  Stats: 20 lines in 2 files changed: 3 ins; 7 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/206.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/206/head:pull/206

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/206