RFR: Reset coalesce and fill boundary during global collect

[Kelvin Nilsen]

On Thu, 14 Oct 2021 22:27:49 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Normally, this happens during cset selection for the old generation. It also needs to happen for coalesce and fill at the end of global collections or objects may not be filled.

I'm actually not sure this is the right way to fix this.  The implementation of ShenandoahHeapRegion::oop_fill_and_coalesce() does not re-register surviving objects.  It only registers the result of coalescing previously independent objects. 

If we feel a need for this fix, maybe the bug is in the implementation of heap->card_scan()->coalesce_objects().

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PR: https://git.openjdk.java.net/shenandoah/pull/90