RFR: 8343500: Optimize ArrayClassDescImpl computeDescriptor

[Shaojin Wen]

A small improvement to ArrayClassDescImpl#computeDescriptor that avoids intermediate object allocation in the common rank 1 scenario.

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Commit messages:
 - optimize computeDescriptor

Changes: https://git.openjdk.org/jdk/pull/21860/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21860&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343500
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21860.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21860/head:pull/21860

PR: https://git.openjdk.org/jdk/pull/21860