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

[via llvm-commits]

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@@ -692,6 +692,20 @@ class CastInst : public UnaryInstruction {
   }
 };
 
+/// Instruction that can have a nneg flag (only zext).
+class NonNegInstruction : public CastInst {
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goldsteinn wrote:

Personally not a fan of `NonNegInstruction` as the name. To me atleast, that seems to imply a lot more coverage than just `zext`.
Maybe a bit more specific `NonNegCastInstruction`?

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