Move complex pred modes for keyframes to s2
Move complex pred modes for keyframes to s2

Currently the speed gap for speeds 1-3 is very unbalanced.
Speed 2 is only 20% faster than speed 1, while speed 3 is 150% faster
than speed 2. After this change, speed 2 becomes 50% faster than speed
1, and speed 3 becomes 50% faster than speed 2. Impact on metrics at
speed 2 is very small: [AWCY](https://bit.ly/3q0sJrl)

xiph/rav1e,xiph/rav1e