avx2: signed extensions (#132)
avx2: signed extensions (#132)

_mm256_cvtepi8_epi16
_mm256_cvtepi8_epi32
_mm256_cvtepi8_epi64
_mm256_cvtepi16_epi32
_mm256_cvtepi16_epi64
_mm256_cvtepi32_epi64
rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch,rust-lang/stdarch