[RISCV] Handle .vx pseudos in hasAllNBitUsers (#67419)

Vector pseudos with scalar operands only use the lower SEW bits (or less in the
case of shifts and clips). This patch accounts for this in hasAllNBitUsers for
both SDNodes in RISCVISelDAGToDAG. We also need to handle this in
RISCVOptWInstrs otherwise we introduce slliw instructions that are less
compressible than their original slli counterpart.

This is a reland of aff6ffc8 with the
refactoring omitted.