sha/asm/keccak1600-avx2.pl: remodel register usage.

This gives much more freedom to rearrange instructions. This is
unoptimized version, provided for reference. Basically you need
to compare it to initial 29724d0e15b4934abdf2d7ab71957b05d1a28256
to figure out the key difference.

Reviewed-by: Rich Salz <rsalz@openssl.org>