RFR: 8342119: Shenandoah: Remove extra ShenandoahUpdateRefsClosure

[Aleksey Shipilev]

On Tue, 15 Oct 2024 08:53:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We have a duplicate `ShenandoahUpdateRefsClosure`, which we use in a few places. For bulk GC operations, we have `ShenandoahConcUpdateRefsClosure` and `ShenandoahSTWUpdateRefsClosure` that are semantically similar to that one. We can drop the `ShenandoahUpdateRefsClosure` in favor of these two existing ones. 
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
>  - [x] Performance tests (no regressions)

Extended performance tests showed ever-so-slight regression in thread root times with concurrent closure. So new revision rolls that back, and uses the similar non-concurrent closure we used before. I also renamed a few things to make it very explicit where we are using non-concurrent closures, even in concurrent mode.

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PR Comment: https://git.openjdk.org/jdk/pull/21517#issuecomment-2419666178