[llvm] [LangRef] make consequences of NaN rules for pow(i) more explicit (PR #170177)

[Ralf Jung via llvm-commits]

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@@ -16831,6 +16836,11 @@ trapping or setting ``errno``.
 When specified with the fast-math-flag 'afn', the result may be approximated
 using a less accurate calculation.
 
+Note that due to how :ref:`LLVM treats NaN values <floatnan>`, the special cases
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RalfJung wrote:

When those documents talk about "NaN", they mean "quiet NaN" (similar to how some documents say that fmin/fmax return the non-NaN argument if the other one is NaN, but only refer to quiet NaNs when they say that). Some `pow` implementations return `QNaN` for `pow(1, SNaN)`; for instance, the one that comes with the GNU libc. musl returns `1` for the same inputs. (See https://rust.godbolt.org/z/chsbv5v4d.)

https://github.com/llvm/llvm-project/pull/170177