arm64: enable test cases for:
uqshrn{2}, sqrshrun{2}, sqshrun{2} (vector, imm)
sqxtn{2}, uqxtn{2}, sqxtun{2} (vector and scalar)


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14257 a5019735-40e9-0310-863c-91ae7b9d1cf9