[llvm] [VPlan] Remove no-op SCALAR-STEPS after unrolling. (PR #123655)

[Florian Hahn via llvm-commits]

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@@ -924,6 +924,17 @@ static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  // VPScalarIVSteps can only be simplified after unrolling. VPScalarIVSteps for
+  // part 0 can be replaced by their start value, if only the first lane is
+  // demanded.
+  if (auto *Steps = dyn_cast<VPScalarIVStepsRecipe>(&R)) {
+    if (Steps->getParent()->getPlan()->isUnrolled() && Steps->isPart0() &&
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fhahn wrote:

In general it is the other way around, unrolling introduces copies of the input recipe, one for each part. So only after unrolling recipes for different 'parts' (i.e. interleaved iterations) are introduced.

https://github.com/llvm/llvm-project/pull/123655