RFR: 8255991: Shenandoah: Apply 'weak' LRB on cmpxchg and xchg

[Aleksey Shipilev]

On Fri, 6 Nov 2020 17:45:53 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> It is possible to access a Reference's referent by using various cmpxchg and xchg intrinsics. When that happens, we need to apply the weak LRB to prevent resurrection.
> 
> Testing: hotspot_gc_shenandoah

Makes sense to me. Looks good.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1098