Unnecessary scaling in ivas_core_dec_fx()

Bug description

This is a follow up to !2191 (merged)..

There is unnecessary scaling back and forth of the buffer hb_synth_32_fx[] in function ivas_core_dec_fx(), see details in !2191 (comment 86681). Consequently, a precision is reduced due to the back (32bit -> 16bit) and forth (16bit -> 32bit) scaling.

It is very likely that a similar issue exists for other buffers as well. From a more general point of view, duplication of the same buffer (like hb_synth_16_fx[] and hb_synth_32_fx[] in this case ) complicates the framework at many places and increases its complexity and memory, and should be avoided.