[RISCV] Group the legal vector types into lists we can iterator over in the...

[RISCV] Group the legal vector types into lists we can iterator over in the RISCVISelLowering constructor

Remove the RISCVVMVTs namespace because I don't think it provides
a lot of value. If we change the mappings we'd likely have to add
or remove things from the list anyway.

Add a wrapper around addRegisterClass that can determine the
register class from the fixed size of the type.

Reviewed By: frasercrmck, rogfer01

Differential Revision: https://reviews.llvm.org/D95491