[RISCV][InsertVSETVLI] Relax tail policy more often for vmv.s.x

If a vm.s.x pseudo has an undef passthru operand, then we're free to use
whatever tail policy we want for VL > 1. We previously relaxed the tail
policy for this but only when we could also expand the SEW.
This patch changes it to relax the tail policy even if the SEW can't be
expanded and removes a few more toggles, as well as fully moving the
vmv.s.x logic into getDemanded.