[DAGCombiner] Bail out if vector size is not a multiple

For the included test case, the DAG transformation
  concat_vectors(scalar, undef) -> scalar_to_vector(sclr)
would attempt to create a v2i32 vector for a v9i8
concat_vector.  Bail out to avoid creating a bitcast with
mismatching sizes later on.

Differential Revision: https://reviews.llvm.org/D42379

llvm-svn: 323312