[llvm] [DAGCombiner] Attempt to fold 'add' nodes to funnel-shift or rotate (PR #125612)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> Can we update comments to show the `add` cases too? For example,
> 
> ```
>   // fold (or (shl x, (*ext y)),                                                 
>   //          (srl x, (*ext (sub 32, y)))) ->                                    
>   //   (rotl x, y) or (rotr x, (sub 32, y))
> ```

I've updated all comments to reflect the add case as well.

https://github.com/llvm/llvm-project/pull/125612