RFR (S) 8244509: Shenandoah: refactor ShenandoahBarrierC2Support::test_* methods

[Roman Kennke]

Looks ok to me.

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8244509
> 
> These methods are parts of critical paths and should be written in easy to understand form. Makes
> follow-up patches much more obvious. This one is mostly mechanical refactoring, except for
> introducing and rewiring "old_ctrl".
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8244509/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, benchmarks
>