[RISCV] Use multiclass inheritance to simplify some of riscv_vector.td. NFCI

We don't need to instantiate single multiclasses inside of
other multiclasses. We can use inheritance and save writing 'defm ""'.

Reviewed By: khchen

Differential Revision: https://reviews.llvm.org/D100074

GitOrigin-RevId: 5f6b3d1833fdf20e18a3c8bf3ede7b5060130f73
1 file changed