Function is_ascii 

const fn is_ascii(bytes: &[u8]) -> bool

Available on x86-64 and target feature sse2, or LoongArch LA64 and target feature lsx only.

ASCII test optimized to use the pmovmskb instruction on x86-64 and the vmskltz.b instruction on loongarch64.

Other platforms are not likely to benefit from this code structure, so they use SWAR techniques to test for ASCII in usize-sized chunks.