[llvm] [InstCombine] Canonicalize `icmp ult (add X, C2), C` expressions (PR #95649)

[via llvm-commits]

goldsteinn wrote:

NB regarding the proofs, you can just do `%C2_cnt = call i8 @llvm.ctpop.i8(i8 %C2); %C2_is_pow2 = icmp eq i8 %C2_cnt, 1` for power of 2 lemma.

https://github.com/llvm/llvm-project/pull/95649