Resolve "Improve WMOPS performance of IGF_getWhiteSpectralData_ivas()"

Closes #2029 (closed)

For the testcases

./IVAS_cod -ism_sba 4 3 scripts/testv/stvISM1.csv scripts/testv/stvISM2.csv scripts/testv/stvISM3.csv scripts/testv/stvISM4.csv 128000 48 scripts/testv/stvOSBA_4ISM_3OA48c.wav ../../stvOSBA_4ISM_3OA48c.bit
./IVAS_dec binaural 48 ../../stvOSBA_4ISM_3OA48c.bit eins.wav
./IVAS_dec 7_1_4 48 ../../stvOSBA_4ISM_3OA48c.bit zwei.wav

The WMOPS performance improved from

                                 total  1000.00  377.837 401.922 388.294
                                 total  1000.00  364.774 391.736 377.125

To

                                 total  1000.00  376.409 400.606 387.126
                                 total  1000.00  363.298 390.422 375.957