RFR: Merge shenandoah:genshen [v3]

[Roman Kennke]

On Fri, 12 Feb 2021 19:42:05 GMT, earthling-amzn <github.com+71722661+earthling-amzn at openjdk.org> wrote:

>>> Taking a look now. The bot suggests changing the title of this PR to indicate that it's a merge, otherwise it will be squashed. If this gets squashed, won't it make subsequent merges from master more difficult? Is it okay with you if I change the title of this PR so this goes into a `genshen` as a merge and not a giant squashed commit?
>> 
>> Yes, please!
>
> The bot says only 'known authors' can create merge requests.

I've integrated this merge directly. Seemed the easiest approach :-)

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PR: https://git.openjdk.java.net/shenandoah/pull/14