[llvm] [VPlan] Unroll VPReplicateRecipe by VF. (PR #142433)

[via llvm-commits]

================
@@ -936,6 +936,13 @@ class VPInstruction : public VPRecipeWithIRFlags,
     BranchOnCount,
     BranchOnCond,
     Broadcast,
+    /// Creates a struct of fixed-width vectors containing all operands. The
+    /// number of operands
+    /// matches the number of fields in the struct.
----------------
ayalz wrote:

```suggestion
    /// Given operands of (the same) struct type, creates a struct of fixed-
    /// width vectors each containing a struct field of all operands. The
    /// number of operands matches the element count of every vector.
```
(Strictly speaking, the vectors created contain the fields of the operands, rather than the complete operands, in contrast to BuildVector.)

https://github.com/llvm/llvm-project/pull/142433