[lworld] RFR: 8375328: [lworld] JNI IsSameObject function doesn't use the alternate substitutability method

[Frederic Parain]

Small fix to align the implementation of JNI function IsSameObject with the implementation of `if_acmp<cond>` in the rest of the JVM.

Tested locally.

Fred

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Commit messages:
 - Add support for the alrternate substitutability method

Changes: https://git.openjdk.org/valhalla/pull/1905/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1905&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375328
  Stats: 11 lines in 2 files changed: 10 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla/pull/1905.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1905/head:pull/1905

PR: https://git.openjdk.org/valhalla/pull/1905