[non-BE] harmonize-post_decoder_ivas_fx-and-post_decoder_fx

• Related issues: #2429
• Requested reviewers: @multrus

Reason why this change is needed

• Harmonize post_decoder_ivas_fx() and post_decoder_fx() (if possible)

Description of the change

• Non-BE change: take over ZIR blending from EVS and IVAS Floating-point code; this was not identical already in EVS FX.

Affected operating points

• Affects few IVAS operating points (all BR switching), where post_decoder_fx() was used previously. Diffs by this change are small, however or sometimes triggering larger diffs later (e.g. for ParamISM, MASA or JBM)
• Bitstreams are fully interoperable, but decoder output differs slightly.