RFR[XS] JDK-8178543 - Optimize Klass::is_shared()

[Ioi Lam]

Hi, please review this small start-up enhancement:

https://bugs.openjdk.java.net/browse/JDK-8178543
http://cr.openjdk.java.net/~iklam/jdk10/8178543-klass-is-shared.v01/

We have a benchmark that shows Klass::is_shared() is called very 
frequently during InstanceKlass::link_class_impl, and costs about 2% of 
the start-up time.

The fix is simple -- instead of walking the list of CDS shared regions, 
use a new bit in Klass::_access_flags

Thanks
- Ioi