[llvm] Introducing a new ISD::POISON SDNode to represent the poison value in the IR. (PR #125883)

[Amy Kwan via llvm-commits]

================
@@ -1130,6 +1130,9 @@ class SelectionDAG {
     return getNode(ISD::UNDEF, SDLoc(), VT);
   }
 
+  /// Return an POISON node. POISON does not have a useful SDLoc.
----------------
amy-kwan wrote:

```suggestion
  /// Return a POISON node. POISON does not have a useful SDLoc.
```

https://github.com/llvm/llvm-project/pull/125883