tcg/riscv: Implement vector cmp/cmpsel ops

tcg/riscv: Implement vector cmp/cmpsel ops

Extend comparison results from mask registers to SEW-width elements,
following recommendations in The RISC-V SPEC Volume I (Version 20240411).
This aligns with TCG's cmp_vec behavior by expanding compare results to
full element width: all 1s for true, all 0s for false.

Signed-off-by: TANG Tiancheng <tangtiancheng.ttc@alibaba-inc.com>
Reviewed-by: Liu Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20241007025700.47259-7-zhiwei_liu@linux.alibaba.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>