[llvm] specify NaN behavior more precisely (PR #66579)

[Jacob Lifshay via llvm-commits]

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@@ -3394,17 +3394,75 @@ Floating-Point Environment
 The default LLVM floating-point environment assumes that traps are disabled and
 status flags are not observable. Therefore, floating-point math operations do
 not have side effects and may be speculated freely. Results assume the
-round-to-nearest rounding mode.
+round-to-nearest rounding mode, and subnormals are assumed to be preserved.
+
+Running LLVM code in an environment where these assumptions are not met can lead
+to undefined behavior. The ``stricfp`` and ``denormal-fp-math`` attributes as
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programmerjake wrote:

```suggestion
to undefined behavior. The ``strictfp`` and ``denormal-fp-math`` attributes as
```

https://github.com/llvm/llvm-project/pull/66579