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

[via llvm-commits]

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@@ -1005,16 +1016,23 @@ void VPlan::execute(VPTransformState *State) {
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, VectorPreHeader, State->CFG.ExitBB}});
 
-  // Replace regular VPBB's for the middle and scalar preheader blocks with
-  // VPIRBasicBlocks wrapping their IR blocks. The IR blocks are created during
-  // skeleton creation, so we can only create the VPIRBasicBlocks now during
-  // VPlan execution rather than earlier during VPlan construction.
+  // Replace regular VPBB's for the vector preheader, middle and scalar
+  // preheader blocks with VPIRBasicBlocks wrapping their IR blocks. The IR
+  // blocks are created during skeleton creation, so we can only create the
+  // VPIRBasicBlocks now during VPlan execution rather than earlier during VPlan
+  // construction.
+  replaceVPBBWithIRVPBB(getVectorPreheader(), VectorPreHeader);
   BasicBlock *MiddleBB = State->CFG.ExitBB;
-  VPBasicBlock *MiddleVPBB = getMiddleBlock();
   BasicBlock *ScalarPh = MiddleBB->getSingleSuccessor();
   replaceVPBBWithIRVPBB(getScalarPreheader(), ScalarPh);
+  VPBasicBlock *MiddleVPBB = getMiddleBlock();
   replaceVPBBWithIRVPBB(MiddleVPBB, MiddleBB);
----------------
ayalz wrote:

```suggestion
  replaceVPBBWithIRVPBB(getVectorPreheader(), VectorPreHeader);
  replaceVPBBWithIRVPBB(getMiddleBlock(), MiddleBB);
  replaceVPBBWithIRVPBB(getScalarPreheader(), ScalarPh);
```
following the logical order listed in the comment? MiddleVPBB can be inlined for consistency.

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