arm64:
* implement: rev32, rev64, saba, uaba, sabd, uabd.
* factor out a large number of duplicated expressions of the form
  bitQ == 0 ? unop(Iop_ZeroHI64ofV128, mkexpr(t)) : mkexpr(t)


git-svn-id: svn://svn.valgrind.org/vex@2892 8f6e269a-dfd6-0310-a8e1-e2731360e62c