RFR(S): 8231055: C2: arraycopy with same non escaping src and dest but different positions causes wrong execution

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8231055/webrev.00/

When a System.arraycopy() has the same source and destination array and
that array doesn't escape, at safepoints we need to record the value of
elements of the array: we can't load element from the source array
(which is also the destination and is going to be eliminated). The
current code doesn't take the source and destination positions into and
records the wrong element values.

Roland.