[llvm] [VPlan] Replace UnrollPart for VPScalarIVSteps with start index op (NFC) (PR #170906)

[via llvm-commits]

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@@ -1526,7 +1526,9 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
   // VPScalarIVSteps for part 0 can be replaced by their start value, if only
   // the first lane is demanded.
   if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(Def)) {
-    if (Steps->isPart0() && vputils::onlyFirstLaneUsed(Steps)) {
+    if ((Steps->getNumOperands() == 3 ||
+         match(Steps->getOperand(3), m_ZeroInt())) &&
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ayalz wrote:

Having 3 operands is fine, trying to see bout the 4th operand, i.e., if `match(Steps->getOperand(3), m_ZeroInt())` can ever be true. OTOH, suggesting above that it should be.

https://github.com/llvm/llvm-project/pull/170906