RFR: 8135078: Refactor InCSetState::is_in_cset_or_humongous

[Erik Helin]

Hi all,

this tiny patch slightly refactors the function
InCSetState::is_in_cset_or_humongous. The new version decouples the
function from the enum representation, this in turn enables future
extensions of the enum wihout affecting the logic. The reason for
InCSetState::is_in_cset_or_humongous was implemented as
`_value != NotInCSet` was for performance reasons, but careful
performance measurements shows that no regressions are introduced with
this patch.

Webrev:
http://cr.openjdk.java.net/~ehelin/8135078/webrev.00/

Testing:
- JPRT

Performance testing:
- SPECjbb2005 with a fixed load to get as stable runs as possible. No
  regression in total gc time nor in object copy time.
- Perf Aurora test run, no regressions

Thanks,
Erik