bd9a290b30
In the `Int8` case of `VECTOR_SH{R,L}_V128`, when all the values are the
same, then a single-instruction `gf2p8affineqb` can be emitted that does
an int8-based arithmetic-shift, utilizing GF(8) arithmetic.
More info here:
https://wunkolo.github.io/post/2020/11/gf2p8affineqb-int8-shifting/
Also fixes the iteration-type for when detecting if all of the simd
lanes are the same value(was iterating `u16` and not `u8`)