[llvm] [LiveDebugVariables] Add basic verification (PR #79846)

[Stephen Tozer via llvm-commits]

================
@@ -43,21 +43,34 @@ class raw_ostream;
   /// SlotIndex & SlotIndexes classes for the public interface to this
   /// information.
   class IndexListEntry : public ilist_node<IndexListEntry> {
-    MachineInstr *mi;
+#if NDEBUG
+    // Disable poison checks such that setPoison will do nothing and isPoisoned
+    // will return false.
+    static constexpr unsigned PoisonBits = 0;
+    static constexpr unsigned PoisonVal = 0;
+#else
+    static constexpr unsigned PoisonBits = 1;
+    static constexpr unsigned PoisonVal = 1;
+#endif
+
+    PointerIntPair<MachineInstr *, PoisonBits, unsigned> mi;
----------------
SLTozer wrote:

```suggestion
    PointerIntPair<MachineInstr *, PoisonBits> mi;
```
Nit: The default type is `unsigned`, so we can skip explicitly passing it.

https://github.com/llvm/llvm-project/pull/79846