[llvm] [VPlan] Fold FirstOrderRecurrenceSplice(X, X) to X when X is uniform (PR #171772)

[Paul Walker via llvm-commits]

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@@ -1449,6 +1449,14 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  // Replace splice(A, A, -1) with A if A is uniform.
+  if (match(Def, m_VPInstruction<VPInstruction::FirstOrderRecurrenceSplice>(
+                     m_VPValue(A), m_Deferred(A))) &&
+      vputils::isSingleScalar(A)) {
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paulwalker-arm wrote:

Does that mean only the function description needs updating? e.g.
"Returns true if \p VPV is a single scalar, because it produces the same value for all lanes, and potentially only has its first lane used."


https://github.com/llvm/llvm-project/pull/171772