[RISCV] Relax some Zvbb patterns and lowerings to Zvkb (#68115)

vandn, vrev8 and vro{l,r} are now part of Zvkb, which Zvbb now implies.
This
patch updates the predicates to check for Zvkb instead of Zvbb in the
tablegen
patterns for the SD and VL nodes, as well as some of the lowering logic
in
RISCVISelLowering.