[llvm] [VPlan] Split out optimizeEVLMasks. NFC (PR #174925)

[Florian Hahn via llvm-commits]

================
@@ -266,6 +266,14 @@ struct VPlanTransforms {
   addExplicitVectorLength(VPlan &Plan,
                           const std::optional<unsigned> &MaxEVLSafeElements);
 
+  /// Optimize recipes which use an EVL based header mask to a VP intrinsic:
+  ///
+  /// %mask = icmp step-vector, EVL
----------------
fhahn wrote:

```suggestion
  /// %mask = icmp ult step-vector, EVL
```
?

https://github.com/llvm/llvm-project/pull/174925