[llvm] [VPlan] Move tail folding out of VPlanPredicator. NFC (PR #176143)

[Andrei Elovikov via llvm-commits]

================
@@ -427,14 +427,44 @@ struct VPlanTransforms {
   static void narrowInterleaveGroups(VPlan &Plan, ElementCount VF,
                                      TypeSize VectorRegWidth);
 
+  /// Adapts the vector loop region for tail folding by introducing a header
+  /// mask and predicating the region:
+  ///
+  /// Vector loop region before:
+  /// +-------------------------------------------+
+  /// |%iv = ...                                  |
+  /// |...                                        |
+  /// |%iv.next = add %iv, vfxuf                  |
+  /// |branch-on-count %iv.next, vector-trip-count|
+  /// +-------------------------------------------+
+  ///
+  /// Vector loop region after:
+  /// +-------------------------------------------+
+  /// |%iv = ...                                  |
+  /// |%wide.iv = widen-canonical-iv ...          |
+  /// |%header-mask = icmp ult %wide.iv, BTC      |
+  /// |branch-on-cond %header-mask                |---+
+  /// +-------------------------------------------+   |
+  ///                      |                          |
+  ///                      v                          |
+  /// +-------------------------------------------+   |
+  /// |                   ...                     |   |
+  /// +-------------------------------------------+   |
+  ///                      |                          |
+  ///                      v                          |
+  /// +-------------------------------------------+   |
+  /// |%iv.next = add %iv, vfxuf                  |<--+
----------------
eas wrote:

```suggestion
  /// <new phis>                                  |<--+
  /// |%iv.next = add %iv, vfxuf                  |
```

https://github.com/llvm/llvm-project/pull/176143