Miri ICEs on non-power-of-2 SIMD vectors

[RalfJung]

#![feature(core_intrinsics)]
#![feature(repr_simd)]

use core::intrinsics::simd::*;

#[repr(simd)]
struct Simd<T, const N: usize>(pub [T; N]);

fn main() { unsafe {
    let v1 = Simd([1u32; 3]);
    let v2 = Simd([2u32; 3]);
    let v3 = simd_add(v1, v2);
    assert_eq!(v3.0, [3u32; 3]);
} }

Cc rust-lang/portable-simd#319