AArch64: print NEON lists with a space.

This matches ARM64 behaviour, which I think is clearer. It also puts all the
churn from that difference into one easily ignored commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207116 91177308-0d34-0410-b5e6-96231b3b80d8