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

[Aleksey Shipilev]

> 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20215/files
  - new: https://git.openjdk.org/jdk/pull/20215/files/21433847..ca1ad60e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20215&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20215&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20215.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20215/head:pull/20215

PR: https://git.openjdk.org/jdk/pull/20215