RFR: Abandon old gen cset candidates upon global collection

[Roman Kennke]

On Thu, 13 May 2021 18:31:59 GMT, William Kemper <wkemper at openjdk.org> wrote:

> When a _global_ collection occurs, it will include the old generation. When this happens, it doesn't make sense to continue with mixed collections of the old generation. This changes fixes an assert caused by the same old region being added more than once to the collection set.

Looks good! Thank you!

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Marked as reviewed by rkennke (Lead).

PR: https://git.openjdk.java.net/shenandoah/pull/38