[llvm] [InstCombine] Fold `icmp samesign u{gt/lt} (X +nsw C2), C` -> `icmp s{gt/lt} X, (C - C2)` (PR #169960)

[Tirthankar Mazumder via llvm-commits]

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@@ -3183,6 +3183,18 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
       return new ICmpInst(Pred, X, ConstantInt::get(Ty, NewC));
   }
 
+  CmpInst::Predicate ChosenPred = Pred.getPreferredSignedPredicate();
+
+  if (Pred.hasSameSign() && Add->hasNoSignedWrap() &&
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wermos wrote:

It seems to me that the `samesign` flag is required for the transformation to be correct. See [this](https://alive2.llvm.org/ce/z/WzjQ5M) Alive2 link for a counterexample.

Since the `samesign` seems to be required, I don't know if this modification should be done.

https://github.com/llvm/llvm-project/pull/169960