RFR: 8364501: Compiler shutdown crashes on access to deleted CompileTask [v2]

[Aleksey Shipilev]

> See the bug for more investigation. 
> 
> In short, with recent changes to `delete` `CompileTask`-s, we end up in the rare situation where we can access tasks that have been already deleted. The major and obivous mistake I committed myself with [JDK-8361752](https://bugs.openjdk.org/browse/JDK-8361752) in `CompileQueue::delete_all`: the code first `delete`-s, then asks for `next` (facepalms). 
> 
> Another case is less trivial, and mostly fix in abundance of caution: in `wait_for_completion`, we can exit while blocking task is still in queue. Current code skip deletions only when compiler is shutdown for compilation, but I think the condition should be stronger: unless the task is completed, we should assume it might carry the queue-ing `next`/`prev` pointers that `delete_all` would need, and skip deletion. Realistically, it would "leak" only on compiler shutdown, like before.
> 
> I have also put in some diagnostic code to catch the lifecycle issues like this more reliably, and cleaned up `next`, `prev` lifecycle to clearly disconnect the `CompileTasks` that are no longer in queue.
> 
> Additional testing:
>  - [x] Linux AArch64 server fastdebug, reproducer no longer fails
>  - [x] Linux AArch64 server fastdebug, `compiler`
>  - [x] Linux AArch64 server fastdebug, `all`

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

  Chicken out of memset-ing the possibly vtable-bearing object

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/26696/files
  - new: https://git.openjdk.org/jdk/pull/26696/files/e5f0a180..64d0b3c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26696&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26696&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26696.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26696/head:pull/26696

PR: https://git.openjdk.org/jdk/pull/26696