[RISCV] Fix wrong operand being used for VL in shift combine

At some point a merge operand was added to the binary vl ops, so this combine
was using the mask for the VL. This causes a crash when trying to
select the vmv_v_x_vl, which showed up locally when messing about with
selectVSplat, but thankfully in ToT the vmv_v_x_vl gets pattern matched
away into the .vx and .vi operands every time, so there's no noticeable
change.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D158634