[lworld] RFR: 8364191: [lworld] Accesses to atomic flat fields prevent scalar replacement [v2]

[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 incrementally with one additional commit since the last revision:

  small fix StoreFlatNode::size_of

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1518/files
  - new: https://git.openjdk.org/valhalla/pull/1518/files/23828cc7..428b656c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1518&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1518&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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