[lworld] RFR: 8364191: [lworld] Accesses to atomic flat fields prevent scalar replacement [v4]

[Quan Anh Mai]

> Hi,
> 
> Flat accesses prevent scalar replacement because they are mismatched accesses. It is also generally not possible to look through them, because the payload may contain an oop in the form of raw bits. As a result, this PR adds `LoadFlatNode` and `StoreFlatNode`, which act as high-level abstractions for atomic accesses on flat fields. When it is determined that there is no racing access on the flat field (e.g. because the holder object does not escape), these flat access nodes can be expanded into multiple accesses to each flattened fields, otherwise, they will be expanded into a sequence of inferring a payload and accessing memory with that payload.
> 
> I also fix an issue with deoptimization reallocation where we miss assigning the null marker of elements in a nullable flat array.
> 
> Please take a look and leave your reviews, thanks a lot.

Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains eight commits:

 - Several fixes
 - fix gc barrier
 - Merge branch 'lworld' into flatfield
 - fix merge, assert GC barriers
 - Merge branch 'lworld' into flatfield
 - fix release build
 - small fix StoreFlatNode::size_of
 - add LoadFlatNode and StoreFlatNode

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Changes: https://git.openjdk.org/valhalla/pull/1518/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1518&range=03
  Stats: 989 lines in 17 files changed: 775 ins; 188 del; 26 mod
  Patch: https://git.openjdk.org/valhalla/pull/1518.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1518/head:pull/1518

PR: https://git.openjdk.org/valhalla/pull/1518