[llvm] [IR] Add new function attribute nocreateundeforpoison (PR #164809)

[Matt Arsenault via llvm-commits]

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@@ -2741,6 +2741,10 @@ For example:
 ``"nooutline"``
     This attribute indicates that outlining passes should not modify the
     function.
+``nocreateundeforpoison``
+    This attribute indicates that the result of the function will not be undef
+    or poison if all arguments are not undef and not poison. Otherwise, it is
+    undefined behavior.
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arsenm wrote:

I think phrasing it in terms of what it doesn't mean is worse. Can we phrase it in terms of... base value behavior? 

https://github.com/llvm/llvm-project/pull/164809