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

[Andy Kaylor via llvm-commits]

andykaylor wrote:

> max/min operations claim that it should return `either` operand, so we shouldn't return `+0` for max(-0, -0).

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.

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