Performs an affine transformation on the packed bytes in x. That is computes a*x+b over the Galois Field 2^8 for each packed byte with a being a 8x8 bit matrix and b being a constant 8-bit immediate value. Each pack of 8 bytes in x is paired with the 64-bit word at the same position in a.
Uses the writemask in k - elements are zeroed in the result if the corresponding mask bit is not set. Otherwise the computation result is written into the result.