[llvm] [DAG] fold `avgs(sext(x), sext(y))` -> `sext(avgs(x, y))` (PR #95365)

[via llvm-commits]

c8ef wrote:

> > [DAG] fold avgu(sext(x), sext(y)) -> sext(avgu(x, y))
> 
> No, this needs to be: avg**s**(sext(x), sext(y)) -> sext(avg**s**(x, y))

Sorry, my bad.

https://github.com/llvm/llvm-project/pull/95365