RFR (S) 8242365: Shenandoah: use uint16_t instead of jushort for liveness cache

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242365

I was actually looking into this weird statement:
  size_t max = (1 << (sizeof(jushort) * 8)) - 1;

...which is borderline UB, because we can shift for more than datatype width. Then I figured we
might as well replace jushort with uint16_t.

Webrev:
  https://cr.openjdk.java.net/~shade/8242365/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey