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

[Nikita Popov via llvm-commits]

================
@@ -12900,7 +12903,8 @@ Example:
 
 .. code-block:: llvm
 
-      %X = select i1 true, i8 17, i8 42          ; yields i8:17
+      %X = select i1 true, i8 17, i8 42                   ; yields i8:17
+      %Y = select nnan i1 true, float 0.0, float NaN      ; yields float:0.0
----------------
nikic wrote:

```suggestion
      %Y = select nnan i1 true, float 0.0, float NaN      ; yields float:0.0
      %Z = select nnan i1 false, float 0.0, float NaN     ; yields float:poison
```

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