RFR: 8337981: ShenandoahHeap::is_in should check for alive regions [v2]

[Aleksey Shipilev]

> The expected behavior of `CollectedHeap::is_in` is to check whether the object belongs to the committed parts of the heap:
> https://github.com/openjdk/jdk/blob/d19ba81ce12a99de1114c1bfe67392f5aee2104e/src/hotspot/share/gc/shared/collectedHeap.hpp#L273-L276
> 
> This is useful to check if object resides in the parts of the heap the GC knows are not dead. Yet, Shenandoah's check just verifies that oop is within the heap bounds. So `is_in` check for an object that is in trashed/empty region would pass by accident, and we will miss detecting bugs. This should be rectified. I believe "committed" is too weak for the test as well, since we really want to know if we can touch the object, i.e. if it is in active region.
> 
> I re-wired assertions/verification code to be clear whether we check for heap bounds or actual in-heap conditions.
> 
> Deeper testing revealed that reference processing code potentially loads a dead referent, but only to null-check it, or ask bitmap about it. Still, more precise `in_heap` check fails asserts in `CompressedOops::decode`. That required a bit of touchup as well.
> 
> Additional testing:
>  - [x] Linux AArch64 server fastdebug, `all` with `-XX:+UseShenandoahGC`

Aleksey Shipilev has updated the pull request incrementally with three additional commits since the last revision:

 - Style touchups
 - Fixing ShenandoahReferenceProcessor
 - Verifier fix

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20492/files
  - new: https://git.openjdk.org/jdk/pull/20492/files/dbab6d43..69c66853

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20492&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20492&range=00-01

  Stats: 35 lines in 2 files changed: 22 ins; 7 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/20492.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20492/head:pull/20492

PR: https://git.openjdk.org/jdk/pull/20492