Teach DAG combine to fold (extract_subvec (concat v1, ..) i) to v_i

Teach DAG combine to fold (extract_subvec (concat v1, ..) i) to v_i

- If the extracted vector has the same type of all vectored being concatenated
  together, it should be simplified directly into v_i, where i is the index of
  the element being extracted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@166125 91177308-0d34-0410-b5e6-96231b3b80d8