The encryption and decryption code paths are mostly identical, except
for a small difference where the plaintext input into the MAC is taken
from either the input or the output block.

We can factor this in quite easily using a vector bit select, and a few
additional XORs, without the need for branches. This way, we can use the
same tail handling logic on the encrypt and decrypt code paths, allowing
further consolidation of the asm helpers in a subsequent patch.

(In the main loop, adding just a handful of ALU instructions results in
a noticeable performance hit [around 5% on Apple M2], so those routines
are kept separate)
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>