Add a dagcombine optimization to convert concat_vectors of undefs into a single undef.

The unoptimized concat_vectors isd prevented the canonicalization of the vector_shuffle node.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160221 91177308-0d34-0410-b5e6-96231b3b80d8