[AArch64] Improve SAD pattern

Given a vecreduce_add node, detect the below pattern and convert it to the node
sequence with UABDL, [S|U]ADB and UADDLP.

i32 vecreduce_add(
 v16i32 abs(
   v16i32 sub(
    v16i32 [sign|zero]_extend(v16i8 a), v16i32 [sign|zero]_extend(v16i8 b))))
=================>
i32 vecreduce_add(
  v4i32 UADDLP(
    v8i16 add(
      v8i16 zext(
        v8i8 [S|U]ABD low8:v16i8 a, low8:v16i8 b
      v8i16 zext(
        v8i8 [S|U]ABD high8:v16i8 a, high8:v16i8 b

Differential Revision: https://reviews.llvm.org/D104042