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

[Artem Belevich via llvm-commits]

https://github.com/Artem-B approved this pull request.

LGTM in principle, but please wait if someone else wants to chime in.

If there's no objection, I'd suggest landing the patch early next week and watch the bots in case something unexpected pops up. 

I'm not sure if there are existing tests covering add/shift on other platforms. Given that there are no changes in existing tests that code path may not have enough coverage. It may be useful to add the tests for NVPTX and some common targets first, so we can observe the effects of this patch. It's much easier to reason about the impact of the patch when the test diffs show what changed, as opposed to the new state of affairs only.

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