arm64: implement
  sshr, ushr, ssra, usra (scalar, imm) 
  srshr, urshr, srsra, ursra (scalar, imm) 
  srshr, urshr, srsra, ursra (vector, imm) 
  sshl, srshl, ushl, urshl (scalar, imm) 
  sshl, srshl, ushl, urshl (vector, vector) 
  ssra, usra (vector, imm) 


git-svn-id: svn://svn.valgrind.org/vex/trunk@2926 8f6e269a-dfd6-0310-a8e1-e2731360e62c