Part 2 of memcpy-x86 optimization 

This is an additional optimization of memcpy-x86 for the tail end of 7,6,5,4 quadwords (56,48,40,32 bytes) On a micro benchmark this change produces a speedup of 20-40% in memcpy throughput. It might affect the other cases depending upon the code prefetched into L1, L2 cache. If the percentage of JITed code is significant this second order affect shouldn't arise.