Add a simple folding rule for Iop_ZeroHI64ofV128.


git-svn-id: svn://svn.valgrind.org/vex@2921 8f6e269a-dfd6-0310-a8e1-e2731360e62c
arm64: implement: {uqshl, sqshl, sqshlu} (vector, imm).


git-svn-id: svn://svn.valgrind.org/vex@2922 8f6e269a-dfd6-0310-a8e1-e2731360e62c