[Complexity] Optimize ivas_dirac_dec_decorr_process_fx()

Basic info

This is a sub-task of issue #1010 (closed) on optimization of ivas_dirac_dec_decorr_process_fx(). The current implementation of ivas_dirac_dec_decorr_process_fx() has a complexity of 228.587 WMOPS for the testcase as described in #1010 (closed).. The proposed optimization reduces this to 128.807 WMOPS.

Bug description

Ways to reproduce

See commandline in #1010 (closed).