cmd/compile: take into account that floating point special cases (NaN, Inf) are unlikely

[mundaym]

math.IsNaN and math.IsInf are very unlikely to return true. It would be nice to use this information for code layout.

I have hacked together a prototype using SSA rules but there is probably a better way to do it. The prototype is here: https://go-review.googlesource.com/c/go/+/64191. Feel free to take that and improve it.

[josharian]

Haven't looked at the CL, but naively it seems like the likelyadjust pass is the right place to do this.

[bcmills]

I would certainly believe that math.IsNaN is usually false. I suspect that math.IsInf depends on the application.