RFR: 8256999: Add C2 intrinsic for Reference.refersTo and PhantomReference::refersTo

[Aleksey Shipilev]

On Wed, 25 Nov 2020 03:31:36 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

> JDK-8188055 added the function Reference.refersTo. For performance, the supporting native methods Reference.refersTo0 and PhantomReference.refersTo0 should be intrinsified by C2.
> 
> Initial patch was prepared by @fisk.
> 
> Tested hs-tier1-4. Added new compiler tests to test intrinsics.
> 
> Ran new test with Shenandoah. Found only one issue. As result I disable  PhantomReference::refersTo intrinsic for COOP+ Shenandoah combination. Someone from Shenandoah team have to test changes if that is enough.

I just pulled the fresh master, applied this patch on top, enabled `_PhantomReference_refersTo0` in `c2compiler.cpp`, and ran `CONF=linux-x86_64-server-fastdebug make images run-test TEST=tier1 TEST_VM_OPTS="-XX:+UseShenandoahGC"` without problems. 

@vnkozlov, what Shenandoah failure did you see? Attention @rkennke.

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PR: https://git.openjdk.java.net/jdk/pull/1425