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

[Artem Belevich via llvm-commits]

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@@ -1836,6 +1836,12 @@ and non-0 as true.
   This operator produces 1 if *a* is not the uninitialized value (``?``) and 0
   otherwise.
 
+``!instances<``\ *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. The format of *regex* is ERE (Extended POSIX Regular
+    Expressions).
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Artem-B wrote:

At which point in time will the existence of those instances be considered?

See more of my questions in the test file below.

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