Missed optimization with reserve and extend_from_slice

[SUPERCILEX]

I tried this code:

pub fn zzz(v: &mut Vec<u8>, s: &[u8]) {
    v.reserve(s.len());
    v.extend_from_slice(s);
}

I expected to see this happen: only one do_reserve_and_handle call.

Instead, this happened: extend_from_slice stills checks the capacity and calls reserve. It seems like LLVM assumes aren't able to bubble up out of multiple functions. I was able to get the optimization to work in #119465 but it required putting assumes in every function along the reserve chain.