AES-NI/OCB: Optimize last and first key XORing

* cipher/rijndael-aesni.c (aesni_ocb_enc, aesni_ocb_dec)
[__x86_64__]: Reorder and mix first and last key XORing with OCB offset
XOR operations.

OCB pre-XORing and post-XORing can be mixed and reordered with
first and last round XORing of AES cipher. This commit utilizes
this fact for additional optimization of AES-NI/OCB encryption
and decryption.

Benchmark on Intel Haswell:

Before:
AES | nanosecs/byte mebibytes/sec cycles/byte auto Mhz

OCB enc |     0.174 ns/B      5468 MiB/s     0.697 c/B      3998
OCB dec |     0.170 ns/B      5617 MiB/s     0.679 c/B      3998

After (enc ~11% faster, dec ~6% faster):
AES | nanosecs/byte mebibytes/sec cycles/byte auto Mhz

OCB enc |     0.157 ns/B      6065 MiB/s     0.629 c/B      3998
OCB dec |     0.160 ns/B      5956 MiB/s     0.640 c/B      3998

For reference, CTR:
AES | nanosecs/byte mebibytes/sec cycles/byte auto Mhz

CTR enc |     0.157 ns/B      6090 MiB/s     0.626 c/B      3998
CTR dec |     0.157 ns/B      6092 MiB/s     0.626 c/B      3998

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>