RFR: 8312262: Klass::array_klass() should return ArrayKlass pointer

[Coleen Phillimore]

This is a simple change to make array_klass() return ArrayKlass (the first dimension of TypeArrayKlass is a TypeArrayKlass so can't use ObjArrayKlass), higher_dimension is always an ObjArrayKlass and lower_dimension can be a TypeArrayKlass.  The change removes some casts.
Tested with tier1 linux/macosx/windows on x86 and aarch64.

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Commit messages:
 - 8312262: Klass::array_klass() should return ArrayKlass pointer

Changes: https://git.openjdk.org/jdk/pull/15059/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15059&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312262
  Stats: 61 lines in 11 files changed: 1 ins; 5 del; 55 mod
  Patch: https://git.openjdk.org/jdk/pull/15059.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15059/head:pull/15059

PR: https://git.openjdk.org/jdk/pull/15059