[llvm] [LangRef] Clarify the semantics of fast-math flags (PR #89442)

[Matt Arsenault via llvm-commits]

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@@ -3656,6 +3656,45 @@ floating-point transformations.
 ``fast``
    This flag implies all of the others.
 
+When performing a transformation that involves more that one instruction, the
+flags required to enable the transformation must be set on all instructions
+involved in the transformation, and any new instructions created by the
+transformation should have only those flags set which were set on all the
+original instructions that are being transformed.
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arsenm wrote:

If you want to add more of a guideline, maybe could say something about how you can infer that. e.g. since nnan/ninf imply something about the inputs in one instruction, it implies something about the value in the other use 

https://github.com/llvm/llvm-project/pull/89442