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

[Chen Liang]

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

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Commit messages:
 - 8372729

Changes: https://git.openjdk.org/valhalla/pull/1790/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1790&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372729
  Stats: 63 lines in 2 files changed: 63 ins; 0 del; 0 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