[lworld] RFR: 8372729: [lworld] Incorrect result of acmp with flat, nullable value class field [v3]

[Chen Liang]

> LayoutIteration is missing null markers. Added the test from tobias to SubstitutabilityTest.

Chen Liang has updated the pull request incrementally with three additional commits since the last revision:

 - Improve tests
 - Fix null marker detection
 - Revert "Need to add delta"
   
   This reverts commit 18e5c26f258b0436a952275a2593a933e34dac31.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1790/files
  - new: https://git.openjdk.org/valhalla/pull/1790/files/18e5c26f..99c3c615

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1790&range=02
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1790&range=01-02

  Stats: 104 lines in 2 files changed: 77 ins; 21 del; 6 mod
  Patch: https://git.openjdk.org/valhalla/pull/1790.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1790/head:pull/1790

PR: https://git.openjdk.org/valhalla/pull/1790