Even without SIMD, the "naive" version would compile into using two conditional move instructions, the way the attempted optimization ends up with a conditional

Even without SIMD, the "naive" version would compile into using two conditional move instructions, the way the attempted optimization ends up with a conditional move in one of its branches. SIMD or not, the superiority of the "naive" version is the sort of outcome you could guess, if you were better at guessing.