[llvm] [LVI] Learn value ranges from ctpop/ctlz/cttz results (PR #121945)

[Yingwei Zheng via llvm-commits]

================
@@ -1159,6 +1162,29 @@ getRangeViaSLT(CmpInst::Predicate Pred, APInt RHS,
   return std::nullopt;
 }
 
+/// Get value range for a "ctpop(Val) Pred RHS" condition.
+ValueLatticeElement
+LazyValueInfoImpl::getValueFromICmpCtpop(ICmpInst::Predicate Pred, Value *RHS) {
+  unsigned BitWidth = RHS->getType()->getPrimitiveSizeInBits();
+
+  auto RHSConst = dyn_cast<ConstantInt>(RHS);
+  if (!RHSConst)
+    return ValueLatticeElement::getOverdefined();
+
+  auto &RHSVal = RHSConst->getValue();
+
+  ConstantRange ResValRange = ConstantRange::makeExactICmpRegion(Pred, RHSVal);
+
+  unsigned ResMin = ResValRange.getUnsignedMin().getLimitedValue(BitWidth);
+  unsigned ResMax = ResValRange.getUnsignedMax().getLimitedValue(BitWidth);
+
+  APInt ValMin, ValMax;
+  APInt AllOnes = APInt::getAllOnes(BitWidth);
+  ValMin = AllOnes.lshr(BitWidth - ResMin);
+  ValMax = AllOnes.shl(BitWidth - ResMax);
----------------
dtcxzyw wrote:

```suggestion
  APInt ValMin = APInt::getLowBitsSet(BitWidth, ResMin);
  APInt ValMax = APInt::getHighBitsSet(BitWidth, ResMax);
```

https://github.com/llvm/llvm-project/pull/121945