[llvm] [RISCV][WIP] Optimize sum of absolute differences pattern. (PR #82722)

[Philip Reames via llvm-commits]

preames wrote:

> Posting for discussion.

Well, I'd been planning on posting a similar patch and was trying to figure out how to motivate it cleanly enough to have you approve it.  :)

My recent set of narrowing patches were motivated by the same workload, thought they do apply a bit more broadly across the different kernels.  My thinking was that I'd start with the slightly broader applicability pieces, then return to this one once the low hanging fruit was done.  

https://github.com/llvm/llvm-project/pull/82722