Function safe_arch::copy_i64_m128i_s

#[must_use]pub fn copy_i64_m128i_s(a: m128i) -> m128i

This is supported with target feature sse2 only.

Copy the low i64 lane to a new register, upper bits 0.

let a = m128i::from([1_i64, 2]);
let b = copy_i64_m128i_s(a);
assert_eq!(<[i64; 2]>::from(b), [1, 0]);