Harmonize tcx_ltp_pitch_search_*(), tcx_ltp_find_gain_*fx()
# Bug description
Seems the function pairs
* `tcx_ltp_pitch_search()` and `tcx_ltp_pitch_search_ivas_fx()`
* `tcx_ltp_find_gain()` and `tcx_ltp_find_gain_ivas_fx()`
could be harmonized in a BE way with little effort...
issue