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...