[llvm] [DAG]SimplifyDemandedVectorElts-add ISD::AVGCEILS/AVGCEILU/AVGFLOORS/AVGFLOORU nodes (PR #86284)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

If I had to guess the problem is that we don't hit this until after lowering and the AArch64 DUP node combines aren't calling SimplifyDemandedVectorElts - @davemgreen does that make sense to you?

https://github.com/llvm/llvm-project/pull/86284