RFR(L): 6912521: System.arraycopy works slower than the simple loop for little lengths

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/6912521/webrev.00/

Follow up to 6700100 (instance clone as series of loads/stores): convert ArrayCopyNode for small array copies (clone of arrays, System.arraycopy, Arrays.copyOf) to series of loads and stores.

Roland.