Give i32x4 a From impl that uses _mm_set1_epi32

[mooman219]

The impl From<i32> for i32x4 uses new(i32, i32, i32, i32) always. There's an intrinsic to not do that.

impl From<i32> for i32x4 {
  #[inline]
  #[must_use]
  fn from(i: i32) -> Self {
   magic( _mm_set1_epi32(i))
  }
}

[Lokathor]

@mooman219 Thinking about it some more, I don't think this makes a performance difference either way, but if you can show some benchmarks or something then that's cool.