target/riscv: Refactor translation of vector-widening instruction

Zvbb (implemented in later commit) has a widening instruction, which
requires an extra check on the enabled extensions.  Refactor
GEN_OPIVX_WIDEN_TRANS() to take a check function to avoid reimplementing
it.

Signed-off-by: Dickon Hood <dickon.hood@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: <20230711165917.2629866-7-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>