Refactor rd loop for inter modes

This commit pulls the iterative motion search for compound inter-
inter out from handle_inter_mode_ as a separate function. Hence,
it is applicable to 4x4/4x8/8x4 level compound inter search to be
enabled later.

Also edit the rd loop for 4x4 inter block sizes for cosmetic
purpose.

Change-Id: Ibc71a11cbe5a26cd52faba01026cf8446cf4d2b4