Avoid using `Ord::clamp` in the `f16`-specific part of the generic
`scalbn`.
It turned out to be redundant anyway, as both callsites follow a pattern
like
```
if n < negative_val {
let foo = (n + positive_val).clamp(negative_val, positive_val);
}
```
and `n < negative_val < 0` implies `n + positive_val < positive_val`.
Fixes: rust-lang/compiler-builtins#1046
Juho Kahala
5faeb43a5c