[TableGen] Support non-def operators in !getdagop (#77531)

`!getdagop` expects the dag operator to be a def, and errors out if it's
not.

While that's true in most cases, when multiclasses are involved, the
late resolution of the dag operator can result in it not being a def
yet, but still have a proper type, wich is required to check against the
optional parameter Ty in `!getdagop<Ty>`.

e.g, in the following dag:
```
(!cast<TestInstruction>(TestInstructionAndPattern::NAME) foo)
```
the operator is a UnOpInit, but all we need here is to check its type.

This fixes a bug where !getdagop is used to query the dag operator that
is dependent on the multiclass, which is not yet resolved to a def. Once
the folding is performed, the field becomes a record that can be
queried.