[RISCV] Add a test case showing unnecessary vsetvli for mask register instructions.

If the VL argument for a mask instruction comes from a vsetvli with
an SEW!=8, we will insert an extra vsetvli for the mask instruction
even if the SEW/LMUL ratio is the same. This requires at least one
instruction before the mask instruction that needs the SEW of the
explicit vsetvli. Otherwise, we'll just rewrite the explicit vsetvli.