[RISCV] DAG combine (mul (add x, 1), y) -> vmadd (#71495)

vmadd: (mul (add x, 1), y) -> (add (mul x, y), y)
           (mul x, add (y, 1)) -> (add x, (mul x, y))
    vnmsub: (mul (sub 1, x), y) -> (sub y, (mul x, y))
            (mul x, (sub 1, y)) -> (sub x, (mul x, y))
    
    Comparison with gcc:
    vmadd: https://gcc.godbolt.org/z/xjePx87Y7
    vnsub: https://gcc.godbolt.org/z/b17zG7nT1