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

[RogerRiggs]

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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Issue

• JDK-8373930: [lworld] Restore Objects.equals use of identity check (Bug - P4)

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