libvpx

Skip some mode SAD calculation in non-RD mode

This commit checks if the motion vector associated with the current
mode has been computed in previous mode tests. If possible, skip the
redundant reference block generation and SAD calculation in the
non-RD mode decision process.

For test sequence pedestrian_area 1080p, the runtime goes from
24261 ms to 23770 ms. This does not change compression performance.
The speed-up is mostly around places with consistent motion.

Change-Id: I97be63c6a2d07c57be26b3c600fbda3803adddda