[llvm] [LV] Fix handleFirstArgMinOrMax to use 0 as identity element for UMax/SMax (PR #192054)

[Florian Hahn via llvm-commits]

================
@@ -1779,14 +1785,11 @@ static bool handleFirstArgMinOrMax(
     FinalCanIV = DerivedIVRecipe;
   }
 
-  // If the final min/max value matches its start value, the condition in the
-  // loop was always false, i.e. no induction value has been selected. If that's
-  // the case, set the result of the IV reduction to its start value.
-  VPValue *AlwaysFalse = Builder.createICmp(CmpInst::ICMP_EQ, MinOrMaxResult,
-                                            MinOrMaxPhiR->getStartValue());
-  VPValue *FinalIV = Builder.createSelect(
-      AlwaysFalse, FindIVSelect->getOperand(2), FinalCanIV);
-  FindIVSelect->replaceAllUsesWith(FinalIV);
+  // Replace the previous FindIV result with the new canonical IV result.
+  // No guard needed: when the condition is never true, FindLastIVPhiR is
+  // initialized with 0, so RawCanIV = 0 and FinalCanIV correctly returns the
+  // start value of the original IV.
+  FindIVSelect->replaceAllUsesWith(FinalCanIV);
----------------
fhahn wrote:

> FinalCanIV = DerivedIV(start_value, 0, step) = start_value, 

That is the start value of the tracked IV, but the start value of the FindIV reduction may be different to that value, as in `@argmin_argmax` for example?

https://github.com/llvm/llvm-project/pull/192054