RFR: 8336635: Add IR test for Reference.refersTo intrinsic [v3]

[Tobias Hartmann]

On Thu, 18 Jul 2024 11:44:53 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> As the follow-up for [JDK-8256999](https://bugs.openjdk.org/browse/JDK-8256999), we can directly test for the (absence of) GC barriers in C2 `Reference.refersTo` intrinsic. This would be come less relevant for G1 after Late Barrier expansion lands, but would still be relevant for Shenandoah for a while. It would also be good for backports.
>> 
>> Additional testing:
>>  - [x] Test fails when I deliberately remove `AS_NO_KEEPALIVE` from the C2 intrinsic
>>  - [x] Test passes with GC explicitly passed as `TEST_VM_OPTS`
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Whitespace

Marked as reviewed by thartmann (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/20215#pullrequestreview-2185586509