[llvm] [IR] Add zext nneg flag (PR #67982)

[Nikita Popov via llvm-commits]

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@@ -11156,6 +11156,9 @@ until it reaches the size of the destination type, ``ty2``.
 
 When zero extending from i1, the result will always be either 0 or 1.
 
+If the ``nneg`` flag is set, and the ``zext`` argument is negative, the result
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nikic wrote:

Done! (I dropped the "semantically equivalent" bit because it's a refinement, not an equivalence.)

https://github.com/llvm/llvm-project/pull/67982