Float16 runtime and compiler intrinsics models do not match

```
julia> f() = Base.fptrunc(Base.Float16, -2.980232238769532e-8)
f (generic function with 1 method)

julia> f()
Float16(-0.0)

julia> f(x) = Base.fptrunc(Base.Float16, x)
f (generic function with 2 methods)

julia> f(-2.980232238769532e-8)
Float16(-6.0e-8)
```