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

[via llvm-commits]

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@@ -473,7 +473,7 @@ class VPBlockBase {
   /// that are actually instantiated. Values of this enumeration are kept in the
   /// SubclassID field of the VPBlockBase objects. They are used for concrete
   /// type identification.
-  using VPBlockTy = enum { VPBasicBlockSC, VPRegionBlockSC };
+  using VPBlockTy = enum { VPBasicBlockSC, VPRegionBlockSC, VPIRBasicBlockSC };
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ayalz wrote:

```suggestion
  using VPBlockTy = enum { VPRegionBlockSC, VPBasicBlockSC, VPIRBasicBlockSC };
```
nit: order more logical?

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