[TableGen] Remove asserts for attributes in aggregate builders

    Attributes can have default values or be optional. Checking the validity of
    attributes in aggregate builder should consider that. And to be accurate,
    we should check all required attributes are indeed provided in the list.
    This is actually duplicating the work done by verifier. Checking the validity
    of attributes should be the responsiblity of verifiers. This CL removes
    the assertion for attributes in aggregate builders for the above reason.
    (Assertions for operands/results are still kept since they are trivial.)

    Also added more tests for aggregate builders.

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PiperOrigin-RevId: 241746059