[Cranelift] Add fmin / fmax variants without NaN propagation

This is required to correctly implement `{f32,f64}::{min,max}` in cg_clif.

Cranelift docs for `fmin` and `fmax`:

> If either operand is NaN, this returns a NaN.

libstd docs for `{f32,f64}::{min,max}`:

> If one of the arguments is NaN, then the other argument is returned.