[RISCV] Store VLMul/NF into RegisterClass's TSFlags

This TSFlags was introduced by https://reviews.llvm.org/D108767.

A base class of all RISCV RegisterClass is added and we store
IsVRegClass/VLMul/NF into TSFlags and add helpers to get them.

This can reduce some lines and I think there will be more usages.

Reviewers: preames, topperc

Reviewed By: topperc

Pull Request: https://github.com/llvm/llvm-project/pull/84894