Ensure loads of vector types have !noundef

[scottmcm]

Assuming that -- like __m128 -- a u32x4 isn't actually allowed to be undef, we should ensure that we have !noundef metadata on the corresponding loads. (This assumes Simd<MaybeUninit<u32>, 4> would be needed for real uninit.)

It looks like today https://rust.godbolt.org/z/hs1MKh15x we don't:

  %_8 = load <2 x i32>, ptr %self, align 8
  %_9 = load <2 x i32>, ptr %other, align 8