RFR: JDK8 C2 fixes

[Roman Kennke]

This change fixes two problems in library_call.cpp:

- in inline_unsafe_access(), read-barriers should be moved up,
otherwise we'd have one store in the else branch that does not have a
read-barrier on its value.

- for arraycopies, we must not turn oop-copies into int-copies, this
would bypass the post-barrier that updates our references.

With those changes, derby passes again without crashing.

Ok?

http://cr.openjdk.java.net/~rkennke/jdk8-c2-fix/webrev.00/

Roman