Only add the option processing and store the result. No attributes are added to FIR yet.
Clang handles this option using -ffp-exception-behavior (and so never passes -fno-trapping-math to the frontend). We don't do that here because gfortran doesn't support -ffp-exception-behavior and I want to keep the scope limited to allow -ffast-math support to progress.
The change to the condition for forwarding -mreassociate is to match Clang.