[AArch64] Handle vector with two different values with efficient vector mask

When we lower BUILD_VECTOR to VECTOR_SHUFFL, we could generate efficient vector
mask. For example,

 t24: v8i8 = BUILD_VECTOR t25, t25, t25, t25, t26, t26, t26, t26
 ==>
   t27: v8i8 = BUILD_VECTOR t26, t26, t26, t26, t26, t26, t26, t26
   t28: v8i8 = BUILD_VECTOR t25, t25, t25, t25, t25, t25, t25, t25
 t29: v8i8 = vector_shuffle<0,1,2,3,12,13,14,15> t27, t2

Differential Revision: https://reviews.llvm.org/D150345