RFR (S) 8244510: Shenandoah: invert SHC2Support::is_in_cset condition

[Roman Kennke]

Looks reasonable.

Thank you,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8244510
> 
> Current code in SHC2Support::is_in_cset treats "in cset" branch as frequent. We should really invert
> it after SFX barriers make that condition much less likely. It improves some benchmarks without any
> observable regressions on others. Note I opted to invert the condition instead of changing the
> probabilities, because I think it flows better: test_in_cset is UNLIKELY to be true now.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8244510/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, benchmarks
>