Draft: Support non-finite checks in math modes with finite assumptions

Compilers often provide a "fast math" option that writes code that makes certain assumptions about values. A common assumption is that the data are always finite. In this case, it is possible that the system operations like std::isnan do not actually check anything but just return false (as the assumption is that the values are finite). This is a problem for code that usually has finite values but may use nonfinite values for flags. We would like to make sure that the values are as expected. This change uses custom comparisons to check for non-finites even in fast math modes (assuming the compiler optimizer does not outsmart us).

Backport: release