arm64: implement: shll #imm, shrn #imm, rshrn #imm,
{smlal,umlal,smlsl,umlsl,smull,umull} (elem)


git-svn-id: svn://svn.valgrind.org/vex@2898 8f6e269a-dfd6-0310-a8e1-e2731360e62c