[lworld] RFR: 8376135: [lworld] Add JIT support for NULLABLE_NON_ATOMIC_FLAT layout [v5]

[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:

  add assert

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1970/files
  - new: https://git.openjdk.org/valhalla/pull/1970/files/f8ec8eb3..2d492a1c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1970&range=04
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1970&range=03-04

  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 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