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

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> > I've also rebased this PR atop that branch so you can see the impact this change would have across those targets.
> 
> For some reason I see only complete test files added. It looks like rebase didn't make it.

The diff on the tests should be viewable within this commit https://github.com/llvm/llvm-project/pull/125612/commits/0f8aef9a5f15d389e24e30e7cd5592fb474eec1e. Does that work? Once I land https://github.com/llvm/llvm-project/pull/132842 the diff will be apparent in the main PR diff. 

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