[lworld] RFR: 8350901: [lworld] Substitutability check is broken for non-flat fields

[Roger Riggs]

Revert "8348904: [lworld] Remove concept of default value object and zeroInstance"

This reverts commit 38041d7572e853348691af5b384a08f245e2354d.

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Commit messages:
 - 8350901: [lworld] Substitutability check is broken for non-flat fields

Changes: https://git.openjdk.org/valhalla/pull/1383/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1383&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350901
  Stats: 109 lines in 13 files changed: 69 ins; 4 del; 36 mod
  Patch: https://git.openjdk.org/valhalla/pull/1383.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1383/head:pull/1383

PR: https://git.openjdk.org/valhalla/pull/1383