[RISCV] Add missing VL arguments to the creation of RISCVISD::VMV_V_X_VL nodes.

VMV_V_X_VL nodes should always have a passthru, a splat, and a VL.
We were sometimes missing the VL.

This went unnoticed because these cases were all selected into the
following node to form a .vx or .vi instruction. The ComplexPattern
that does this, doesn't check the VL operand. I've added an assert
to the ComplexPattern to catch if the operand is missing.

@qcolombet spotted some of these in D134703.