[llvm] [SDAG][tests] add some test cases covering an add-based rotate (PR #132842)

[Alex MacLean via llvm-commits]

AlexMaclean wrote:

> @AlexMaclean Do you need me to commit this?

Thanks! I was distracted by some other work yesterday, just merged. 

https://github.com/llvm/llvm-project/pull/132842