Add a new folding rule:
ZeroHI64ofV128( ZeroHI64ofV128(x) ) --> ZeroHI64ofV128(x)


git-svn-id: svn://svn.valgrind.org/vex@2917 8f6e269a-dfd6-0310-a8e1-e2731360e62c
arm64: implement:
uqshrn{2}, sqrshrun{2}, sqshrun{2} (vector, imm)
sqxtn{2}, uqxtn{2}, sqxtun{2} (vector and scalar)


git-svn-id: svn://svn.valgrind.org/vex@2918 8f6e269a-dfd6-0310-a8e1-e2731360e62c