[llvm] [TableGen] Add `!defined` operator to get defined records (PR #129680)

[Jay Foad via llvm-commits]

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@@ -1722,6 +1722,11 @@ and non-0 as true.
     Example: ``!dag(op, [a1, a2, ?], ["name1", "name2", "name3"])`` results in
     ``(op a1-value:$name1, a2-value:$name2, ?:$name3)``.
 
+``!defined<``\ *type*\ ``>([``\ *regex*\ ``])``
+    This operator produces a list of records whose type is *type*. If *regex*
+    is provided, only records whose name matches the regular expression *regex*
+    will be included.
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jayfoad wrote:

You should document what syntax the regex uses.

https://github.com/llvm/llvm-project/pull/129680