RFR: 8329597: C2: Intrinsify Reference.clear [v9]

[Aleksey Shipilev]

> [JDK-8240696](https://bugs.openjdk.org/browse/JDK-8240696) added the native method for `Reference.clear`. The original patch skipped intrinsification of this method, because we thought `Reference.clear` is not on a performance sensitive path. However, it shows up prominently on simple benchmarks that touch e.g. `ThreadLocal` cleanups. See the bug for an example profile with `RRWL` benchmarks.
> 
> We need to know the actual oop strongness/weakness before we call into C2 Access API, this work models this after existing code for `refersTo0` intrinsics. C2 Access also need a support for `AS_NO_KEEPALIVE` for stores. 
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `all`
>  - [x] Linux AArch64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 18 commits:

 - Simplify: just do keep alive checks
 - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
 - More precise bit-unmasks
 - Reconcile with late barrier expansion in G1
 - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
 - Review comments
 - Merge branch 'master' into JDK-8329597-intrinsify-reference-clear
 - Also dispatch to slow-path on other arches
 - Fix other arches
 - Tighten up comments in Reference javadoc
 - ... and 8 more: https://git.openjdk.org/jdk/compare/580eb62d...9f7ad7ab

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Changes: https://git.openjdk.org/jdk/pull/20139/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20139&range=08
  Stats: 361 lines in 25 files changed: 340 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/20139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20139/head:pull/20139

PR: https://git.openjdk.org/jdk/pull/20139