lavc/flacenc: partially unroll loop in flac_enc_lpc_16

It now does 12 samples per iteration, up from 4.

From 1.8 to 3.2 times faster again.  3.6 to 5.7 times faster overall.
Runtime is reduced by a further 2 to 18%.  Overall runtime reduced by
4 to 50%.

Same conditions as before apply.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>