target/riscv: Move vector translation checks

Move the checks out of `do_opiv{v,x,i}_gvec{,_shift}` functions
and into the corresponding macros. This enables the functions to be
reused in proceeding commits without check duplication.

Signed-off-by: Nazar Kazakov <nazar.kazakov@codethink.co.uk>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Max Chou <max.chou@sifive.com>
Message-ID: <20230711165917.2629866-6-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>