s390: Change insn selection to recognize memcpy-like statements.
Add S390_INCN_MEMCPY and generate MVC for that later on. Saves between 
0.1 - 1.5% of insns. Observed runtime differences on the perf bucket were
within noise margin.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2675 8f6e269a-dfd6-0310-a8e1-e2731360e62c