Refactor 4x4 block level rd loop

This commit makes clean-ups in the rate-distortion loop for 4x4,
4x8, and 8x4 block sizes for the use of iterative motion search.

Removed unnecessary use of bmi in handle_inter_mode.

Deprecated loop over labels in the 4x4/4x8/8x4 block rd search.

Change-Id: I71203dbb68b65e66f073b37abd90d82ef5ae6826