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

[David Sherwood 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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david-arm wrote:

If the plan has been unrolled can this even be part > 0? I just wonder if instead you can simply assert `Steps->isPart0()` is true? Perhaps I'm misunderstanding something, but it sounds like a bug to be requesting part > 0 in a vplan that's already been unrolled.

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