[lworld] RFR: 8376135: [lworld] Add JIT support for NULLABLE_NON_ATOMIC_FLAT layout [v2]

[Quan Anh Mai]

> Hi,
> 
> This PR implements the support of `NULLABLE_NON_ATOMIC_FLAT` layout in the JITs. There is nothing to do in C2. In C1, I need to implement loading/storing nullable value of a non-atomic field.
> 
> The test `TestValueClasses` is failing with `-XX:-TieredCompilation`, which I suspect is due to the substitutability test handling padding bytes incorrectly.
> 
> Please kindly review what there are for now, thanks a lot.

Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:

  Fix packing/unpacking adapters, fix some miscelaneous issues, address comment

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1970/files
  - new: https://git.openjdk.org/valhalla/pull/1970/files/2d28e1cf..bddd1516

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1970&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1970&range=00-01

  Stats: 85 lines in 14 files changed: 19 ins; 22 del; 44 mod
  Patch: https://git.openjdk.org/valhalla/pull/1970.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1970/head:pull/1970

PR: https://git.openjdk.org/valhalla/pull/1970