[DAG][AArch64][ARM] Recognize avg (hadd) from wrapping flags

This slightly extends the creation of hadd nodes to allow them to be generated
with the original type size if wrapping flags allow.
https://alive2.llvm.org/ce/z/bPjakD
https://alive2.llvm.org/ce/z/fa_gzb

Differential Revision: https://reviews.llvm.org/D143371