RFR: 8336635: Add IR test for Reference.refersTo intrinsic

[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`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20215/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20215&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336635
  Stats: 154 lines in 1 file changed: 154 ins; 0 del; 0 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