[llvm] [InstCombine] Fold Carry-Save Adder identities: `(A ^ B) + ((A & B) << 1)` to `A + B` and `(A + B) - ((A & B) << 1)` to `A ^ B` (PR #229511)

[Nikita Popov via llvm-commits]

nikic wrote:

Rejecting due to no evidence of [real-world usefulness](https://llvm.org/docs/InstCombineContributorGuide.html#real-world-usefulness). The PR also fails to meet many other requirements in the InstCombine contributor guide.

https://github.com/llvm/llvm-project/pull/229511