lavc/lpc: exploit even symmetry of window function

Yields 2x improvement in function performance, and boosts aac encoding
speed by ~ 4% overall. Sample benchmark (Haswell+GCC under -march=native):
after:
ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.22s user 0.03s system 105% cpu 4.970 total

before:
ffmpeg -i sin.flac -acodec aac -y sin_new.aac  5.40s user 0.05s system 105% cpu 5.162 total

Reviewed-by: Rostislav Pehlivanov <atomnuker@gmail.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanag@gmail.com>
