[TableGen] Rework verifier generation and error messages

    Previously we bundle the existence check and the MLIR attribute kind check
    in one call. Further constraints (like element bitwidth) have to be split
    into following checks. That is not a nice separation given that we have more
    checks for constraints. Instead, this CL changes to generate a local variable
    for every attribute, check its existence first, then check the constraints.

    Creating a local variable for each attribute also avoids querying it multiple
    times using the raw getAttr() API. This is a win for both performance the
    readability of the generated code.

    This CL also changed the error message to be more greppable by delimiting
    the error message from constraints with boilerplate part with colon.

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PiperOrigin-RevId: 241906132