RFR: 8256106: Bypass intrinsic/barrier when calling Reference.get() from Finalizer [v3]

[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 incrementally with one additional commit since the last revision:

  Rename inactive-getter and improve javadocs

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1140/files
  - new: https://git.openjdk.java.net/jdk/pull/1140/files/61878147..45e35e78

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1140&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1140&range=01-02

  Stats: 12 lines in 2 files changed: 7 ins; 0 del; 5 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