Optimize concat_vectors(X, undef) -> scalar_to_vector(X).
This optimization is not SSE specific so I am moving it to DAGco.
The new scalar_to_vector dag node exposed a missing pattern in the AArch64 target that I needed to add.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193393 91177308-0d34-0410-b5e6-96231b3b80d8