I think this should count as a bug in Rustc. If your compiler generates IR that's just awful enough that it's a coin toss whether the backend can optimize it,

I think this should count as a bug in Rustc. If your compiler generates IR that's just awful enough that it's a coin toss whether the backend can optimize it, whereas it can optimize the "naive" IR you'd write by hand, it's a compiler problem, not a backend problem.