crypto: x86/aes-xts - more code size optimizations

Prefer immediates of -128 to 128, since the former fits in a signed
byte, saving 3 bytes per instruction.  Also prefer VEX-coded
instructions to EVEX where this is easy to do.

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