avcodec/truemotion2: Simplify creating VLC table

avcodec/truemotion2: Simplify creating VLC table

ff_init_vlc_from_lengths() can be used to offload the computation
of the codes; it also allows to omit the check whether the codes
are already properly ordered (they are). In this case, this also allows
to avoid the allocation of the buffer for the codes.

This improves performance: The amount of decicycles for one call to
tm2_build_huff_tables() when decoding tm20.avi from the FATE-suite
decreased from 46239 to 40035. This test consisted of looping 50 times
over the file and iterating the test ten times.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>