[lworld] RFR: 8373930: [lworld] Restore Objects.equals use of identity check

[Roger Riggs]

Revert "8366841: [lworld] Objects.equals without identity comparison".  (Almost all)
Removing the condition on --enable-preview.
Correct the .equals method in test SimplValueGraphs.SimpleValue.

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Commit messages:
 - Restore the mainline implementation of Objects.equal

Changes: https://git.openjdk.org/valhalla/pull/1836/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1836&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373930
  Stats: 10 lines in 2 files changed: 0 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1836.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1836/head:pull/1836

PR: https://git.openjdk.org/valhalla/pull/1836