[lworld] RFR: 8374800: [lworld] Add a NULLABLE_NON_ATOMIC_FLAT layout [v3]

[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 incrementally with one additional commit since the last revision:

  Fix coding style

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1407/files
  - new: https://git.openjdk.org/valhalla/pull/1407/files/91c57ec1..7654118f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1407&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1407&range=01-02

  Stats: 7 lines in 2 files changed: 0 ins; 0 del; 7 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