RFR: 8373367: interp-only mechanism fails to work for carrier threads in a corner case [v10]

[Serguei Spitsyn]

> The `interp-only` mechanism is based on the `JavaThread` objects. Carrier and virtual threads can temporary share the same `JavaThread`. The `java_thread->jvmti_thread_state()` is re-linked to a virtual thread at `mount` and to the carrier thread at `unmount`. The `JvmtiThreadState` has a back link to the `JavaThread` which is also set for virtual thread at a `mount` and carrier thread at an `unmount`. Just one of these two links at the same time is set to the `JavaThread`, the other one has to be set to `nullptr`. The `interp-only` mechanism needs this invariant.
> However, there is a corner case when this invariant is broken. It happens when the `JvmtiThreadState` for carrier thread has just been created. In such case, the link to `JavaThread` is always `non-nullptr` even though a virtual thread is currently mounted on a carrier thread. This simple update fixes the issue in the  `JvmtiThreadState` ctor.
> 
> Also, this update the includes the `interp_only` implementation simplifications and more asserts are added to relevant places. One of the simplification is a removal of the field `JvmtiThreadState::_thread_saved`.
> 
> Testing:
>  - TBD: Mach5 tiers 1-6

Serguei Spitsyn has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 10 commits:

 - resolve merge conflict
 - review: convert a couple of sanity checks into asserts
 - review: restore assert, will remove it as a separate bug fix
 - review: moved comment clarifying _thread from its init to definition
 - review: simplify interp-only; remove _thread_saved; add more asserts
 - Merge
 - review: fixed minor typos in newly added comments
 - review: moved and extended comment in JvmtiThreadState ctor
 - review: some tweaks for carrier thread JvmtiThreadState initialization
 - 8373367: interp-only mechanism fails to work for carrier threads in a corner case

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Changes: https://git.openjdk.org/jdk/pull/29436/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29436&range=09
  Stats: 77 lines in 7 files changed: 26 ins; 20 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/29436.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29436/head:pull/29436

PR: https://git.openjdk.org/jdk/pull/29436