RFR: Don't require barriers for LoadKlass in C2

[Roman Kennke]

Aleksey spotted a diff in sh/jdk vs jdk/jdk yesterday, which appears to
be a left-over from earlier attempts to put the forwarding pointer in
Klass* slot. Back then I needed to force barriers before loading Klass*.
However that is not needed anymore, we should be good loading Klass*
from from-space copy because it is immutable.

http://cr.openjdk.java.net/~rkennke/loadklass-no-barriers/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman