Refactor inter mode rate-distortion search

This commit separates the rate-distortion optimization loop of
superblocks from that of sub8x8 blocks. This allows better design
rate-distortion optimization search loop for each setting. It also
removes the use of SPLITMV and I4X4_PRED therein.

No performance change in speed 0 settings. For bus@CIF at 2000kbps,
the speed 1 runtime goes from 48009ms to 43894ms (about 10% faster).
The overall compression performance on derf changed by -0.021%.

Speed 2 runtime goes from 27114ms to 28700ms (6% slower), while the
overall coding efficiency goes up by 1.629% for derf, 1.236% for yt.

Change-Id: Ie6bdfa0a370148dd60bd800961077f7e97e67dd4