[llvm] [AMDGPU] Remove setcc by using add/sub carryout (PR #155255)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I guess this is fine but I would expect we wouldn't need to reinvent this transform. Actually I just noticed a similar issue due to us not reporting 64-bit carry-out operations as legal, which we could do. Maybe we should try reporting those as legal next? 

https://github.com/llvm/llvm-project/pull/155255