[AArch64][SVE] Add hadd and rhadd support

This adds basic HADD and RHADD support for SVE, by marking the AVGFLOOR
and AVGCEIL as custom and converting those to HADD_PRED/RHADD_PRED
AArch64 nodes. Both the existing intrinsics and the _PRED nodes are then
lowered to the _ZPmZ instructions.

Differential Revision: https://reviews.llvm.org/D131875