[llvm] LangRef: Clarify behaviors of nsz in fast math flag (PR #137567)

[Andy Kaylor via llvm-commits]

andykaylor wrote:

> > You want to restrict it so that it can't return -0 if both arguments are +0. I don't think that's going to happen, but I do think the current definition of nsz allows it.
> 
> In fact it may happen. I had a try to expand `FMINIMUM` with `fadd OP, 0.0`. And finally I found that `FMINIMUM -0.0, -0.0` may return `+0.0`. So I prefer to state explicitly that it is not allowed.

Do you know why it was happening?

https://github.com/llvm/llvm-project/pull/137567