RFR: 8329597: C2: Intrinsify Reference.clear [v8]

[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:
>  - [ ] Linux x86_64 server fastdebug, `all`
>  - [ ] Linux AArch64 server fastdebug, `all`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  More precise bit-unmasks

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20139/files
  - new: https://git.openjdk.org/jdk/pull/20139/files/c3338302..cf808b9a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20139&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20139&range=06-07

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 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