RFR: Fix verifier handling of weak references when scanning the remembered set

[Roman Kennke]

On Wed, 19 May 2021 22:02:11 GMT, William Kemper <wkemper at openjdk.org> wrote:

> In some use cases, `ShenandoahVerifier` is required to _ignore_ weak references (because they will be processed concurrently). Because of the way the remembered set scan handles oops, it must use an `OopIterateClosure` (not a bare `OopClosure`). To make use of the `OopClosure` passed to the remembered set scanner, it was wrapped in an "adapter" that sheared off the `ShenandoahIgnoreReferenceDiscoverer` configured on the passed in closure. This change removes the need to use such an adapter during the verifier's root scan.

Marked as reviewed by rkennke (Lead).

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PR: https://git.openjdk.java.net/shenandoah/pull/41