Function safe_arch::get_i64_from_m128i_s

#[must_use]pub fn get_i64_from_m128i_s(a: m128i) -> i64

This is supported with target feature sse2 only.

Converts the lower lane to an i64 value.

let a = m128i::from([1_i64, 3]);
let b = get_i64_from_m128i_s(a);
assert_eq!(b, 1_i64);