vp9_rd_pick_inter_mode_sb() reorganization

* Reduce the number of short cirtcuit checks by pre-computing and combining like checks.
* Postpone non-trivial initializations until after the shortcircuits are evaluated.
* Add some consts and const pointers.

No change to the actual results of the call or output of the encoder.

Change-Id: Ie44c4702aec6e08cfe0b8b0ba3cd6b57206478d1