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

[Artem Belevich via llvm-commits]

Artem-B 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. 

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