RFR: 8338881: GenShen: Use explicit third temp register for post barrier

[William Kemper]

On Fri, 23 Aug 2024 17:06:56 GMT, William Kemper <wkemper at openjdk.org> wrote:

> * Use register given as `tmp3` argument instead of "out of the blue" `r3`
> * Simplify logic for post barrier call
> 
> ## Testing
> GHA, internal pipelines

Right, Shenandoah doesn't check that the destination for the store now holds a pointer that points into a different region before updating the card table.

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PR Comment: https://git.openjdk.org/shenandoah/pull/486#issuecomment-2307572240