[llvm] [IndVars] Teach widenLoopCompare to use sext if narrow IV is positive and other operand is already sext. (PR #142703)

[Yingwei Zheng via llvm-commits]

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@@ -1630,6 +1630,12 @@ bool WidenIV::widenLoopCompare(WidenIV::NarrowIVDefUse DU) {
 
   // Widen the other operand of the compare, if necessary.
   if (CastWidth < IVWidth) {
+    // If the narrow IV is always postive and the other operand is sext, widen
+    // using sext so we can combine them. This works for all comparison
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dtcxzyw wrote:

```suggestion
    // using sext so we can combine them. This works for all non-signed comparison
```

https://github.com/llvm/llvm-project/pull/142703