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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> If we're saying that, in general, we're not respecting the sign of zero, I don't see why that should be a problem.

The problem we are trying to solve is to say something more specific than this. I assert that if we cannot get the IEEE-754 2008 fuzzy minnum/maxnum behavior by taking a stronger intrinsic with ordered 0 handling, and adding nsz to it, the definition of nsz is broken 

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