Save 2 instructions in vec32cb &load_bits(uint32_t) & co.

[jfaure]

skips setzero() and ~i

// avx512 _mm256_movm_epi8 emulation
__m256i emu_mm256_movm_epi8(uint32_t i) {
  __m256i rep8Bytes = _mm256_shuffle_epi8(_mm256_set1_epi32(i) // dup each byte of i 8 times
     , _mm256_setr_epi64x(0,0x0101010101010101,0x0202020202020202,0x0303030303030303));
  __m256i m = _mm256_set1_epi64x(0x8040201008040201); // repeat 4  [0b10000000 , 0b010000000 .. 0b1]
  __m256i r = _mm256_and_si256(rep8Bytes , m);
  return _mm256_cmpeq_epi8(r , m);
}```