crypto: x86/aes-xts - optimize _compute_first_set_of_tweaks for AVX-512

Optimize the AVX-512 version of _compute_first_set_of_tweaks by using
vectorized shifts to compute the first vector of tweak blocks, and by
using byte-aligned shifts when multiplying by x^8.

AES-XTS performance on AMD Ryzen 9 9950X (Zen 5) improves by about 2%
for 4096-byte messages or 6% for 512-byte messages.  AES-XTS performance
on Intel Sapphire Rapids improves by about 1% for 4096-byte messages or
3% for 512-byte messages.  Code size decreases by 75 bytes which
outweighs the increase in rodata size of 16 bytes.

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