[lworld] RFR: 8351569: [lworld] Revisit atomic access modes in flat var handles [v4]

[Maurizio Cimadamore]

> This PR is an attempt to put var handle support for flat values on a more solid footing. Some more notes in a comment below.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Avoid dynamic var handle creation in the middle of array element access

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1402/files
  - new: https://git.openjdk.org/valhalla/pull/1402/files/1cd81764..70af9a2c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1402&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1402&range=02-03

  Stats: 247 lines in 6 files changed: 142 ins; 30 del; 75 mod
  Patch: https://git.openjdk.org/valhalla/pull/1402.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1402/head:pull/1402

PR: https://git.openjdk.org/valhalla/pull/1402