lavc/flacenc: add sse4 version of the 16-bit lpc encoder

From 1.8 to 2.4 times faster.  Runtime is reduced by 2 to 39%.  The
speed-up generally increases with compression_level.

This lpc encoder is not used with levels < 3 so it provides no speed-up
in these cases.

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