[lworld] RFR: 8279655: [lworld] Bogus error: incompatible types: Object cannot be converted to Foo

[Aggelos Biboudis]

Use of `referenceProjectionOrSelf` in the subtype check of the `areDisjoint` method.

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Commit messages:
 - Use of reference projection in disjointness check

Changes: https://git.openjdk.java.net/valhalla/pull/612/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=612&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279655
  Stats: 19 lines in 2 files changed: 18 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/612.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/612/head:pull/612

PR: https://git.openjdk.java.net/valhalla/pull/612