* Fixes to divsf3
Embarrassingly failed to notice a section that was unchanged from where
it was copied from mulXf3.zig. The test cases for this function series
div{s,d,t}f3 are very incomplete and don't exercise all code paths.
Remove unnecessary switch from divsf3 left during development from when
I tried to make it generic to support f32, f64, and f128 in one go.
Make runtime safety dependent on whether a test is being run.
* divsf3: switch plus to minus
