[lworld] Integrated: 8344454: [lworld] Nullable flat fields fixes

[Frederic Parain]

Various fixes in nullable flat fields:
   - null marker test in the interpreter runtime is incorrect
   - during field layout computation, if the payload is shifted to satisfy alignment requirement, null marker offsets are not updated accordingly

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Commit messages:
 - Various fixes for nullable flat fields

Changes: https://git.openjdk.org/valhalla/pull/1302/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1302&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344454
  Stats: 10 lines in 3 files changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1302.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1302/head:pull/1302

PR: https://git.openjdk.org/valhalla/pull/1302