[AArch64][SVE]: custom lower AVGFloor/AVGCeil.

-Lower AVGFloor(A, B) to:
 SRL(A) + SRL(B) + (A&B)&1.
-Lower AVGCeil(A, B) to:
 SRL(A) + SRL(B) + (A|B)&1.

Differential Revision: https://reviews.llvm.org/D143283