RFR: Also register filler object for retired gclabs

[Roman Kennke]

On Tue, 28 Sep 2021 22:50:55 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Shenandoah verify will complain that the objects created when gclabs are retired are not properly registered. We already have code to register objects when the promotion labs are retired, this change extends it to also cover other gclabs.

Marked as reviewed by rkennke (Lead).

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PR: https://git.openjdk.java.net/shenandoah/pull/74