RFR: 8323552: AbstractMemorySegmentImpl#mismatch returns -1 when comparing distinct areas of the same instance of MemorySegment

[Per Minborg]

This PR proposes to remove an old optimization check that was incorrect making `AbstractMemorySegmentImpl::mismatch` always return `-1` if the source and destination segment are the same (disregarding offsets).

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Commit messages:
 - Remove comment out markings
 - Remove erroneous check

Changes: https://git.openjdk.org/jdk/pull/18426/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18426&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8323552
  Stats: 34 lines in 2 files changed: 27 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18426.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18426/head:pull/18426

PR: https://git.openjdk.org/jdk/pull/18426