This addresses
- request for improvement for faster key setup in RT#3576;
- clearing registers and stack in RT#3554 (this is more of a gesture to
see if there will be some traction from compiler side);
- more commentary around input parameters handling and stack layout
(desired when RT#3553 was reviewed);
- minor size and single block performance optimization (was lying around);
Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 23f6eec71dbd472044db7dc854599f1de14a1f48)
Improve CBC decrypt and CTR by ~13/16%, which adds up to ~25/33%
improvement over "pre-Silvermont" version. [Add performance table to
aesni-x86.pl].
(cherry picked from commit 5599c7331b90d9d29c9914c2a95c16d91485415a)
