Function _mm256_undefined_pd
#[target_feature(enable = "avx")]
pub const fn _mm256_undefined_pd() -> __m256dReturns vector of type __m256d with indeterminate elements.
Despite using the word "undefined" (following Intel's naming scheme), this non-deterministically
picks some valid value and is not equivalent to mem::MaybeUninit.
In practice, this is typically equivalent to mem::zeroed.