RFR: 8273000: Remove WeakReference-based class initialisation barrier implementation

[Mandy Chung]

On Wed, 25 Aug 2021 22:05:24 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> Get rid of WeakReference-based logic in DirectMethodHandle::checkInitialized() and reimplement it with `Unsafe::ensureClassInitialized()`/`shouldBeInitialized()`. 
> 
> The key observation is that `Unsafe::ensureClassInitialized()` does not block the initializing thread. 
> 
> Also, removed `Unsafe::shouldBeInitialized()` in `DMH::shouldBeInitialized(MemberName)` to save on calling into the VM.
> `Unsafe::ensureClassInitialized()` already has a fast-path check which checks whether the class is fully initialized or not.
> 
> Testing: tier1 - tier6

Marked as reviewed by mchung (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/5258