RFR: Various GC cleanups

[Stefan Karlsson]

On Fri, 25 Mar 2022 13:43:57 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Please review these cleanups, mostly done by @stefank.
> 
> It is probably best to review the commits separately.
> 
> Testing: loom-tier1-3
> 
> Regards,
>   Thomas

Marked as reviewed by stefank (no project role).

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PR: https://git.openjdk.java.net/loom/pull/123