The interpolation filter search used to be performed in a code block in
handle_inter_mode(). This change breaks that code out into its own
function to reduce the length of handle_inter_mode and ecapsulate its
functionality, making both functions more readable.

Attention has been paid to make as many arguments constants as can be.

Change-Id: I3fd484137fc0d16a47dba0b18ce0e2b349d24446