swresample/x86/audio_convert: add emms to CONV

Fixes ticket #1874

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit ca2818b88155029bb6f989ee522e7f8e9e9f5927)
Autodetect idcin only if audio properties allow decoding.

Fixes ticket #2688.
(cherry picked from commit 06bede95fcea47d2e51e8ff248c15311f335b898)