RFR: 8336635: Add IR test for Reference.refersTo intrinsic [v4]

[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:

  Better reliability under different compilation regimes

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20215/files
  - new: https://git.openjdk.org/jdk/pull/20215/files/ca1ad60e..407f07ac

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20215&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20215&range=02-03

  Stats: 35 lines in 1 file changed: 18 ins; 7 del; 10 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