The MatrixType, ExtVectorType, VectorSize and AddressSpace attributes
have arguments defined as ExprArguments in Attr.td. So their arguments
should never be ArgIdents and the logic to handle this case can be
removed.
The logic has been replaced by an assertion to ensure the arguments
are always ArgExpressions
I think the asserts aren't necessary, isArgExpr does a ArgsUnion::get<Expr*>, which already asserts.