[llvm] [SandboxVec] Tag insts in a Region with metadata. (PR #109353)

[Alina Sbirlea via llvm-commits]

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@@ -55,8 +56,10 @@ class Region {
   /// vectorization are part of the Region.
   SetVector<Instruction *> Insts;
 
-  /// A unique ID, used for debugging.
-  unsigned RegionID = 0;
+  /// MDNode that we'll use to mark instructions as being part of the region.
+  MDNode *RegionMDN;
+  static constexpr const char *MDKind = "sbvec";
+  static constexpr const char *RegionStr = "region";
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alinas wrote:

```suggestion
  static constexpr const char *RegionStr = "sandboxregion";
```

https://github.com/llvm/llvm-project/pull/109353