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

[Ralf Jung via llvm-commits]

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@@ -3394,17 +3394,41 @@ 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.
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RalfJung wrote:

I made the text mention this attribute now, is that better?

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