RFR: 8367142: Simplify java mirror handling in JNI methods [v3]

[Ioi Lam]

> The purpose of this PR is to simplify JNI code and also to avoid unnecessary `InstanceKlass::cast()` calls by adding a new  function:
> 
> 
> static InstanceKlass* java_lang_Class::as_InstanceKlass(oop java_class);
> 
> 
> This PR is intended to be a strict clean-up that preserves existing behaviors.

Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:

  Removed the (jobject) version of as_Klass/as_InstanceKlass

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27158/files
  - new: https://git.openjdk.org/jdk/pull/27158/files/d943d2fe..f8634eff

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27158&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27158&range=01-02

  Stats: 70 lines in 9 files changed: 4 ins; 13 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/27158.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27158/head:pull/27158

PR: https://git.openjdk.org/jdk/pull/27158