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

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1449,6 +1449,14 @@ static void simplifyRecipe(VPSingleDefRecipe *Def, VPTypeAnalysis &TypeInfo) {
     return;
   }
 
+  // Replace splice(A, A, -1) with A if A is uniform.
----------------
artagnon wrote:

```suggestion
  // splice(A, A, -1) -> A, if A is single-scalar.
```

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