[AArch64] Handle vector with two different values

If vector has two different values and it can be splitted into two sub
vectors with same length, generate two DUP and CONCAT_VECTORS/VECTOR_SHUFFLE.
For example,

 t22: v16i8 = BUILD_VECTOR t23, t23, t23, t23, t23, t23, t23, t23,
                           t24, t24, t24, t24, t24, t24, t24, t24
==>
   t26: v8i8 = AArch64ISD::DUP t23
   t28: v8i8 = AArch64ISD::DUP t24
 t29: v16i8 = concat_vectors t26, t28

Differential Revision: https://reviews.llvm.org/D148347