arm64: route all whole-vector shift/rotate/slice operations
through Iop_SliceV128, so as to give it some testing.  Implement
Iop_SliceV128 in the back end.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2940 8f6e269a-dfd6-0310-a8e1-e2731360e62c