[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout [v4]

[Frederic Parain]

> Strict final instance fields are not subject to concurrent writes during a read access, so they can be flattened even if they are nullable and bigger than 64 bits. The NULLABLE_NON_ATOMIC_FLAT layout is added for this particular case.
> This new layout can also be used in the special case of nullable empty value classes, because their payload contains a single entry, the null-marker, which is naturally atomic.

Frederic Parain has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 17 commits:

 - Fix to work with JDK-8374729
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Fix coding style
 - More renaming
 - Cleanups and fixes
 - Fix Copyright years
 - Fix compilation
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - Improve tests
 - Merge remote-tracking branch 'upstream/lworld' into flat_final
 - ... and 7 more: https://git.openjdk.org/valhalla/compare/8f8398ed...eab559d6

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

Changes: https://git.openjdk.org/valhalla/pull/1407/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1407&range=03
  Stats: 760 lines in 22 files changed: 589 ins; 10 del; 161 mod
  Patch: https://git.openjdk.org/valhalla/pull/1407.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1407/head:pull/1407

PR: https://git.openjdk.org/valhalla/pull/1407