This improves accuracy (very slightly) and speed for processors having
fma3.

Sample benchmark (fate flac-16-lpc-cholesky, Haswell):
old:
5993610 decicycles in ff_lpc_calc_coefs,      64 runs,      0 skips
5951528 decicycles in ff_lpc_calc_coefs,     128 runs,      0 skips

new:
5252410 decicycles in ff_lpc_calc_coefs,      64 runs,      0 skips
5232869 decicycles in ff_lpc_calc_coefs,     128 runs,      0 skips

Tested with FATE and --disable-fma3, also examined contents of
lavu/lls-test.
Reviewed-by: James Almer <jamrial@gmail.com>
Reviewed-by: Henrik Gramner <henrik@gramner.com>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>