crypto: x86/aes-xts - eliminate a few more instructions

- For conditionally subtracting 16 from LEN when decrypting a message
  whose length isn't a multiple of 16, use the cmovnz instruction.

- Fold the addition of 4*VL to LEN into the sub of VL or 16 from LEN.

- Remove an unnecessary test instruction.

This results in slightly shorter code, both source and binary.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>