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

[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.

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Commit messages:
 - Add a ciField for null markers, beautify flattened field name

Changes: https://git.openjdk.org/valhalla/pull/1456/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1456&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356963
  Stats: 100 lines in 6 files changed: 62 ins; 7 del; 31 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