Implement vlseg builtins.
This is just a suggestion, feel free to ignore: a sequence of Ts was easy to parse for the prototype but may be want to consider something like T3v rather than TTTv. I think it would simplify the TString tblgen class and those std::string(N, 'T').
Was this meant to be static here?
We say Tv is invalid, looks like we should check this here and PrintFatalError?
Maybe we can create a new complex type transformer (Tuple:Value), so (Tuple:3) means TTTv. It can avoid to use static here. You could ref https://github.com/llvm/llvm-project/blob/main/clang/utils/TableGen/RISCVVEmitter.cpp#L638