aesni: Use 4-way parallel AES-NI instructions for CBC decryption

aesni: Use 4-way parallel AES-NI instructions for CBC decryption

CBC decryption can be parallelized, and we do so by queueing instructions
to the processor pipeline. While we have enough registers for 128-bit
decryption, the register count is insufficient to hold all variables with
larger key sizes. Nonetheless is 4-way parallelism faster, roughly by ~8%.