Partial fix for #13761.

https://issues.dlang.org/show_bug.cgi?id=13761
Is that possible?  I think `SargNtype` contains the register type for passing, so a struct is "in-memory-only" only when neither are set.  It's also possible that the combination in the other branch will only happen in cases where both are set.  I'm not sure though, it might be best to take this out into another pull and ask Walter.