[LegalizeTypes] In SoftenFloatRes_FNEG, always generate integer arithmetic,...

[LegalizeTypes] In SoftenFloatRes_FNEG, always generate integer arithmetic, never fall back to using fsub.

We would previously fallback if the type wasn't f32/f64/f128. But
I don't think any of the other floating point types ever go through
the softening code anyway. So this code is dead.