[lworld] RFR: 8356963: [lworld] MemNode::optimize_simple_memory_chain fails with "sanity check for basic cases" [v4]

[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 incrementally with one additional commit since the last revision:

  fix SigEntry comparison

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

Changes:
  - all: https://git.openjdk.org/valhalla/pull/1456/files
  - new: https://git.openjdk.org/valhalla/pull/1456/files/f622449e..c7194bf3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1456&range=03
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1456&range=02-03

  Stats: 6 lines in 2 files changed: 3 ins; 1 del; 2 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