[RISCV] Disable hasAllNBitUsers for vector types.

RISCVGenDAGISel.inc can call this before it checks the node type.
Ensure the type is scalar before wasting time to do the more
computationally expensive checks.

This also avoids an assertion if we hit a VMV_X_S instruction
which doesn't have a VL operand which vectorPseudoHasAllNBitUsers
expects.