[llvm] [LangRef] Clarify the behavior of select with FP poison-generating flags (PR #137131)

[Nikita Popov via llvm-commits]

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@@ -12880,7 +12880,10 @@ class <t_firstclass>` type.
    :ref:`fast-math flags <fastmath>`. These are optimization hints to enable
    otherwise unsafe floating-point optimizations. Fast-math flags are only valid
    for selects that return :ref:`supported floating-point types
-   <fastmath_return_types>`.
+   <fastmath_return_types>`. Note that the violation of poison-generating flags on
+   non-selected arm may not result in :ref:`poison <poisonvalues>` return value.
+   For simplicity, if :ref:`fast-math flags <fastmath>` are present, they are only
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nikic wrote:

```suggestion
   If :ref:`fast-math flags <fastmath>` are present, they are only
```

https://github.com/llvm/llvm-project/pull/137131