RFR: 8256106: Bypass intrinsic/barrier when calling Reference.get() from Finalizer [v2]

[Roman Kennke]

> Finalizer calls Reference.get() from the Finalizer to acquire the finalizee. Concurrent reference processing GCs like Shenandoah and ZGC would return NULL for unreachable referents, and thus would not call finalize() on them.
> 
> ZGC works around this by fixing the referent before enqueuing, so that the barrier would take the fast-path, but Shenandoah cannot do this.
> 
> It is ok to bypass the barrier altogether in this place, because the FinalReference is inactive and marking and reference-discovery treat inactive FinalReferences like strong references.
> 
> Testing:
>  - [x] hotspot_gc_shenandoah
>  - [x] tier1 +UseShenandoahGC +ShenandoahVerify
>  - [x] tier2 +UseShenandoahGC +ShenandoahVerify
>  - [x] tier1
>  - [x] tier2

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8256106
 - 8256106: Bypass intrinsic/barrier when calling Reference.get() from Finalizer

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1140/files
  - new: https://git.openjdk.java.net/jdk/pull/1140/files/52820f98..61878147

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1140&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1140&range=00-01

  Stats: 5646 lines in 150 files changed: 3822 ins; 862 del; 962 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1140.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1140/head:pull/1140

PR: https://git.openjdk.java.net/jdk/pull/1140