low bit vectorizer test

Target: X86 / ARM

X86

 -> 32bit  case : OK
 -> 64bit  case : NG
 -> 128bit case : NG

ARM

 -> 16bit  case : OK
 -> 32bit  case : NG
 -> 64bit  case : NG
 -> 128bit case : NG

Reason

X86 has 256bit length register (when AVX2 is enabled).
So we can utilize ymm as v32i8.

On the other hand,
ARM don't have wide length register.

I will try to improve this.