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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey