Optimize vector8_has_le() on AArch64.

Optimize vector8_has_le() on AArch64.

Presently, the SIMD implementation of this function uses unsigned
saturating subtraction to find bytes less than or equal to the
given value, which is a workaround for the lack of unsigned
comparison instructions on some architectures.  However, Neon
offers vminvq_u8(), which returns the minimum (unsigned) value in
the vector.  This commit adds a Neon-specific implementation that
uses vminvq_u8() to optimize vector8_has_le() on AArch64.

In passing, adjust the SSE2 implementation to use vector8_min() and
vector8_eq() to find values less than or equal to the given value.
This was the only use of vector8_ssub(), so it has been removed.

Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/aNHDNDSHleq0ogC_%40nathan