target/riscv: Move gen_* helpers for RVB

target/riscv: Move gen_* helpers for RVB

Move these helpers near their use by the trans_*
functions within insn_trans/trans_rvb.c.inc.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210823195529.560295-11-richard.henderson@linaro.org
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>