[llvm] [VPlan] Hook IR blocks into VPlan during skeleton creation (NFC) (PR #114292)

[via llvm-commits]

================
@@ -3810,39 +3812,42 @@ class VPlan {
   DenseMap<const SCEV *, VPValue *> SCEVToExpansion;
 
 public:
-  /// Construct a VPlan with original preheader \p Preheader, trip count \p TC,
-  /// \p Entry to the plan and with \p ScalarHeader wrapping the original header
-  /// of the scalar loop. At the moment, \p Preheader and \p Entry need to be
-  /// disconnected, as the bypass blocks between them are not yet modeled in
-  /// VPlan.
-  VPlan(VPBasicBlock *Preheader, VPValue *TC, VPBasicBlock *Entry,
-        VPIRBasicBlock *ScalarHeader)
-      : VPlan(Preheader, Entry, ScalarHeader) {
+  /// Construct a VPlan with \p Entry entering the plan, trip count \p TC and
+  /// with \p ScalarHeader wrapping the original header of the scalar loop.
+  VPlan(VPBasicBlock *Entry, VPValue *TC, VPIRBasicBlock *ScalarHeader)
+      : VPlan(Entry, ScalarHeader) {
     TripCount = TC;
   }
 
-  /// Construct a VPlan with original preheader \p Preheader, \p Entry to
-  /// the plan and with \p ScalarHeader wrapping the original header of the
-  /// scalar loop. At the moment, \p Preheader and \p Entry need to be
-  /// disconnected, as the bypass blocks between them are not yet modeled in
-  /// VPlan.
-  VPlan(VPBasicBlock *Preheader, VPBasicBlock *Entry,
-        VPIRBasicBlock *ScalarHeader)
-      : Entry(Entry), Preheader(Preheader), ScalarHeader(ScalarHeader) {
+  /// Constructor variants that take disconnected preheader and entry blocks,
+  /// connecting them as part of construction.
+  /// FIXME: Only used to reduce the need of code changes during transition.
+  VPlan(VPBasicBlock *OriginalPreheader, VPValue *TC,
+        VPBasicBlock *EntryVectorPreHeader, VPIRBasicBlock *ScalarHeader);
+  VPlan(VPBasicBlock *OriginalPreheader, VPBasicBlock *EntryVectorPreHeader,
+        VPIRBasicBlock *ScalarHeader);
+
+  /// Construct a VPlan with \p Entry to the plan and with \p ScalarHeader
+  /// wrapping the original header of the scalar loop.
+  VPlan(VPBasicBlock *Entry, VPIRBasicBlock *ScalarHeader)
+      : Entry(Entry), ScalarHeader(ScalarHeader) {
     Entry->setPlan(this);
-    Preheader->setPlan(this);
-    assert(Preheader->getNumSuccessors() == 0 &&
-           Preheader->getNumPredecessors() == 0 &&
-           "preheader must be disconnected");
     assert(ScalarHeader->getNumSuccessors() == 0 &&
            "scalar header must be a leaf node");
   }
 
   ~VPlan();
 
+  void setEntry(VPBasicBlock *VPBB) {
+    Entry = VPBB;
+    VPBB->setPlan(this);
+  }
+
   /// Create initial VPlan, having an "entry" VPBasicBlock (wrapping
-  /// original scalar pre-header ) which contains SCEV expansions that need
-  /// to happen before the CFG is modified; a VPBasicBlock for the vector
+  /// original scalar pre-header) which contains SCEV expansions that need
+  /// to happen before the CFG is modified (when executing a VPlan for the
+  /// epilogue vector loop, the original entry needs to be replaced by the new
+  /// entry for the epilogue vector loop); a VPBasicBlock for the vector
----------------
ayalz wrote:

```suggestion
  /// epilogue vector loop, the original entry needs to be replaced by a new
  /// one); a VPBasicBlock for the vector
```

https://github.com/llvm/llvm-project/pull/114292