Draft: Fix #15639 - %inf*%i returns a different result than %i*%inf under Linux/macOS.

Issue fixed by OpenBLAS:

• https://github.com/OpenMathLib/OpenBLAS/issues/4413
• https://github.com/OpenMathLib/OpenBLAS/issues/86
• https://github.com/OpenMathLib/OpenBLAS/issues/4726
• https://github.com/OpenMathLib/OpenBLAS/issues/4728
• https://github.com/OpenMathLib/OpenBLAS/issues/4747

This also adds an extensive test of borderline cases and removes dead code.