LLVM is not evaluating X u > C, a, b the same way it evaluates X <= C, b, a.
To fix this, let's move the folds to after the canonicalization of -1 to TrueVal.
Let's allow splat vectors with poison elements to be recognized too!
Finally, for completion, handle the one case that isn't caught by the above checks because it is canonicalized to eq:
X == -1 ? -1 : X + 1 -> uadd.sat(X, 1)
Alive2 Proof:
https://alive2.llvm.org/ce/z/WEcgYH
ede49fa5f3