Use `_mm256_bslli_epi128`

[alantudyk]

_mm256_bslli_epi128 compiles to the same assembler but the name is more descriptive than _mm256_slli_si256:

algorithmica/content/english/hpc/algorithms/prefix.md

Line 103 in ed1945c

x = _mm256_add_epi32(x, _mm256_slli_si256(x, 4));