RFR (sh/jdk11): Better abstraction for C2 arraycopy LRB

[Roman Kennke]

This makes the arraycopy LRB insertion in sh/jdk11 a little more clear 
by moving the actual code into ShBSC2.

http://cr.openjdk.java.net/~rkennke/shjdk11-arraycopy-lrb/webrev.00/

OK?

Roman