RFR: 8316427: Duplicated code for {obj,type}ArrayKlass::array_klass

[Coleen Phillimore]

Please review this trivial change to move duplicated array_klass().  Tested with tier1 on Oracle supported platforms.

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Commit messages:
 - 8316427: Duplicated code for {obj,type}ArrayKlass::array_klass

Changes: https://git.openjdk.org/jdk/pull/15791/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15791&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316427
  Stats: 198 lines in 6 files changed: 65 ins; 130 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15791.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15791/head:pull/15791

PR: https://git.openjdk.org/jdk/pull/15791