[llvm] InstSimplify: teach simplifyICmpWithConstant about samesign (PR #125899)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -3012,7 +3012,7 @@ static Value *simplifyICmpWithConstant(CmpPredicate Pred, Value *LHS,
   }
 
   // Rule out tautological comparisons (eg., ult 0 or uge 0).
-  ConstantRange RHS_CR = ConstantRange::makeExactICmpRegion(Pred, *C);
+  ConstantRange RHS_CR = ConstantRange::makeAllowedICmpRegion(Pred, *C);
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artagnon wrote:

So, there is a problem with that, because samesign ranges are fundamentally asymmetric; the following assert fails:

```
  assert(makeAllowedICmpRegion(Pred, C) == makeSatisfyingICmpRegion(Pred, C));
```

which leads to the expectation in callers being broken, so that code like this:

```
  const auto CR1 = ConstantRange::makeExactICmpRegion(Pred1, *RHSC1);
  const auto CR2 = ConstantRange::makeExactICmpRegion(Pred2, *RHSC2);

  return CR1.inverse() == CR2;
```

(ValueTracking.cpp:8617)

regresses, even if the assert is removed.

Not sure if there is some better solution to the situation, but my proposal is to have makeAllowedICmpRegion region respect samesign, and makeExactICmpRegion not respect it.

https://github.com/llvm/llvm-project/pull/125899