[llvm] [llvm][TableGen] Add a !initialized predicate to allow testing for ? (PR #117964)

[Akshat Oke via llvm-commits]

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@@ -1815,6 +1816,10 @@ and non-0 as true.
   ``int``. If the result is not 0, the *then* expression is produced; otherwise
   the *else* expression is produced.
 
+``!initialized(``\ *a*\ ``)``
+  This is produces a 1 if *a* is not the unitialized value (``?``) and 0
+  otherwise.
+
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optimisan wrote:

minor grammar tweak:
```suggestion
``!initialized(``\ *a*\ ``)``
  This operator produces 1 if *a* is not the uninitialized value (``?``) and 0
  otherwise.

```

https://github.com/llvm/llvm-project/pull/117964