[llvm] [indvars] Support known positive extends in getExtendedOperandRecurrence (PR #70990)

[Nikita Popov via llvm-commits]

================
@@ -1384,7 +1384,22 @@ WidenIV::getExtendedOperandRecurrence(WidenIV::NarrowIVDefUse DU) {
   else if (ExtKind == ExtendKind::Zero && OBO->hasNoUnsignedWrap())
     ExtendOperExpr = SE->getZeroExtendExpr(
       SE->getSCEV(DU.NarrowUse->getOperand(ExtendOperIdx)), WideType);
-  else
+  else if (DU.NeverNegative) {
----------------
nikic wrote:

I would write the code here like this, to avoid duplicating the logic:
```
  if (OBO->hasNoSignedWrap() && (ExtKind == ExtendKind::Sign || DU.NeverNegative)) {
    ExtKind = ExtendKind::Sign;
    ExtendOperExpr = SE->getSignExtendExpr(
SE->getSCEV(DU.NarrowUse->getOperand(ExtendOperIdx)), WideType);
  } else if (OBO->hasNoUnsignedWrap() && (ExtKind == ExtendKind::Zero || DU.NeverNegative)) {
    ExtKind = ExtendKind::Zero;
    ExtendOperExpr = SE->getZeroExtendExpr(
SE->getSCEV(DU.NarrowUse->getOperand(ExtendOperIdx)), WideType);
  } else
    return {nullptr, ExtendKind::Unknown};
```

https://github.com/llvm/llvm-project/pull/70990