[RISCV] Fix mistakes in fixed-vectors-vreductions-mask.ll command lines. NFC

There were 4 RUN lines, but only 2 of them were unique. I believe
we were trying to test LMUL=1 and LMUL=8 with riscv32 and riscv64.
But put riscv32 on both LMUL=1 lines and riscv64 on both LMUL=8 lines.