[lworld] RFR: 8356963: [lworld] MemNode::optimize_simple_memory_chain fails with "sanity check for basic cases" [v3]

[Quan Anh Mai]

> Many thanks to @TobiHartmann for the speedy investigation. This patch fixes the issue by adding a `ciField` for the null marker. I also do a small change to make the name of the `ciField` of a flattened field clearer. I think adding a scenario to test flattening of nullable and atomic fields is necessary. There are some failures that I filed https://bugs.openjdk.org/browse/JDK-8357061 for the investigation.
> 
> Please kindly review, thanks a lot.

Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'lworld' into nullmarker
 - better error messages
 - include
 - Add a ciField for null markers, beautify flattened field name

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1456/files
  - new: https://git.openjdk.org/valhalla/pull/1456/files/128b8292..f622449e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1456&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1456&range=01-02

  Stats: 33445 lines in 662 files changed: 16829 ins; 14349 del; 2267 mod
  Patch: https://git.openjdk.org/valhalla/pull/1456.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1456/head:pull/1456

PR: https://git.openjdk.org/valhalla/pull/1456