[llvm] [VPlan] Use VPlan predecessors in VPWidenPHIRecipe (NFC). (PR #126388)

[via llvm-commits]

================
@@ -1958,13 +1958,12 @@ class VPScalarPHIRecipe : public VPHeaderPHIRecipe {
 #endif
 };
 
-/// A recipe for handling phis that are widened in the vector loop.
-/// In the VPlan native path, all incoming VPValues & VPBasicBlock pairs are
-/// managed in the recipe directly.
+/// A recipe for widened phis. Incoming values are operands of the recipe and
+/// their operand index corresponds to the incoming predeocessor block. If the
+/// recipe is placed in an entry block to a (non-replicate) region, it must have
+/// exactly 2 incoming values, from from the predecessors of the region and one
+/// from the exiting block of the region.
----------------
ayalz wrote:

```suggestion
/// exactly 2 incoming values, the first from the predecessors of the region and the second
/// from the exiting block of the region.
```


https://github.com/llvm/llvm-project/pull/126388