[RISCV] Use multiclass inheritance where possible for the VPat* multiclasses...

[RISCV] Use multiclass inheritance where possible for the VPat* multiclasses in RISVInstrInfoVPseudos. NFCI

Instead of instantiating multiclasses inside multiclasses, just
inherit from them.

We can do the same for the VPseudo* multiclasses, but that may
interfere with the scheduler class work.