Helps to get rid of MOV and generating constants for each iteration.
```
name old time/op new time/op delta
Sum/AVX2Inline_digest-8 1.57ms ± 2% 1.41ms ± 0% -10.52% (p=0.000 n=9+9)
name old speed new speed delta
Sum/AVX2Inline_digest-8 63.6MB/s ± 1% 71.1MB/s ± 0% +11.76% (p=0.000 n=9+9)
```
Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>