[AArch64] Use intermediate step for concat_vectors of illegal truncs.

Optimize concat_vectors of truncated vectors, where the intermediate
type is illegal, to avoid said illegality,  e.g.,
  (v4i16 (concat_vectors (v2i16 (truncate (v2i64))),
                         (v2i16 (truncate (v2i64)))))
->
  (v4i16 (truncate (v4i32 (concat_vectors (v2i32 (truncate (v2i64))),
                                          (v2i32 (truncate (v2i64)))))))

This isn't really target-specific, and, as such, would best go in the
DAGCombiner.  However, ISD::TRUNCATE legality isn't keyed on both input
and result type, so we might generate worse code when we don't know
better.  On AArch64 we know it's fine for v2i64->v4i16 and v4i32->v8i8.
rdar://20022387

llvm-svn: 232459