RFR (S) 8245465: Shenandoah: test_in_cset can use more efficient encoding

[Aleksey Shipilev]

RFE:
  https://cr.openjdk.java.net/~shade/8245465/webrev.02/

See the rationale in the bug itself.

Webrev:
  https://bugs.openjdk.java.net/browse/JDK-8245465

Testing: hotspot_gc_shenandoah, tier{1,2} with Shenandoah, eyeballing asm, benchmarks

-- 
Thanks,
-Aleksey