[lworld] RFR: 8353195: [lworld] Several failures with -XX:+UseNullableValueFlattening

[Tobias Hartmann]

This PR fixes several issues when running tests with `-XX:+UseNullableValueFlattening` (see JBS description and linked bugs for details).

While testing I found an intermittent SIGSEGV on AArch64 and spent a few days debugging without major progress. It seems to be a GC related issue. I filed [JDK-8355277](https://bugs.openjdk.org/browse/JDK-8355277) for this and will investigate separately.

All compiler tests now pass with `-XX:+UseNullableValueFlattening` on my machine. I'll run more testing with [JDK-8350865](https://bugs.openjdk.org/browse/JDK-8350865).

Thanks,
Tobias

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Commit messages:
 - Cleanup
 - Adjusted comment
 - Merge branch 'lworld' into JDK-8353195
 - Some more fixes
 - Barrier ad file fix
 - More fixes
 - First prototype

Changes: https://git.openjdk.org/valhalla/pull/1428/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1428&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353195
  Stats: 258 lines in 16 files changed: 175 ins; 40 del; 43 mod
  Patch: https://git.openjdk.org/valhalla/pull/1428.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1428/head:pull/1428

PR: https://git.openjdk.org/valhalla/pull/1428