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

[via llvm-commits]

================
@@ -442,6 +442,16 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
   return NewBB;
 }
 
+void VPIRBasicBlock::execute(VPTransformState *State) {
+  assert(getHierarchicalPredecessors().empty() &&
+         "VPIRBasicBlock cannot have predecessors at the moment");
+  assert(getHierarchicalSuccessors().empty() &&
+         "VPIRBasicBlock cannot have successors");
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ayalz wrote:

```suggestion
         "VPIRBasicBlock cannot have successors at the moment");
```
(Pre-preheader should eventually have successors)


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