[llvm] [VPlan] Add VPIRBasicBlock, use to model pre-preheader. (PR #93398)

[via llvm-commits]

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@@ -2948,12 +2953,48 @@ class VPBasicBlock : public VPBlockBase {
     return NewBlock;
   }
 
+protected:
+  void executeRecipes(VPTransformState *State, BasicBlock *BB);
+
 private:
   /// Create an IR BasicBlock to hold the output instructions generated by this
   /// VPBasicBlock, and return it. Update the CFGState accordingly.
   BasicBlock *createEmptyBasicBlock(VPTransformState::CFGState &CFG);
 };
 
+/// A special type of VPBasicBlock that wraps an existing IR basic block.
+/// Recipes of the block get added before the first non-phi instruction in the
+/// wrapped block.
+class VPIRBasicBlock : public VPBasicBlock {
+  BasicBlock *WrappedBlock;
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ayalz wrote:

```suggestion
  BasicBlock *IRBasicBlock;
```
or `IRBB`?


https://github.com/llvm/llvm-project/pull/93398