[X86] Add v8i64->v8i8 ssat/usat/packus truncate tests to min-legal-vector-width.ll

I wonder if we should split the v8i8 stores in order to form
two v4i8 saturating truncating stores. This would remove the
unpckl needed to concatenated the v4i8 results to make a
single store.

llvm-svn: 374519