[llvm] [RISCV] Copy base instruction TSFlags in RVV pseudo TSFlags. NFC (PR #149785)

[Luke Lau via llvm-commits]

lukel97 wrote:

> > To do this I needed to merge Pseudo and RISCVVPseudo so the latter extends the former.
> 
> I support the merging for cleanliness, but I'm not sure what about this change made it necessary.

I may be missing something, but in order to set the `ElementsDependOn`/`DestEEW` fields on Pseudo and access `BaseInstr` from RISCVVPseudo at the same time they needed to be in the same hierarchy. Tablegen complained when I tried to set those fields from RISCVVPseudo otherwise.

I'm going to rework this PR to leave out the TSFlags bit and just have the hierarchy change for now anyway.

https://github.com/llvm/llvm-project/pull/149785