RFR: 8256516: Simplify clearing References

[Aleksey Shipilev]

On Wed, 18 Nov 2020 09:53:11 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this simplification of jlr.Reference clearing by VM code.
> 
> The function java_lang_ref_Reference::set_referent_raw was being used to
> clear the referent of Reference objects, and only for that purpose.  This
> change replaces that function with java_lang_ref_Reference::clear_referent,
> which is much more obvious in intent.  That change is then percolated up
> through callers in the obvious way.
> 
> Testing:
> mach5 tier1

Marked as reviewed by shade (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1286