[llvm][TableGen] Add a !initialized predicate to allow testing for ? (#117964)
There are cases (like in an upcoming patch to MLIR's `Property` class)
where the ? value is a useful null value. However, existing predicates
make ti difficult to test if the value in a record one is operating is ?
or not.
This commit adds the !initialized predicate, which is 1 on concrete,
non-? values and 0 on ?.
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Co-authored-by: Akshat Oke <Akshat.Oke@amd.com>