target/riscv/vector_helper.c: set vstart = 0 in GEN_VEXT_VSLIDEUP_VX()

target/riscv/vector_helper.c: set vstart = 0 in GEN_VEXT_VSLIDEUP_VX()

The helper isn't setting env->vstart = 0 after its execution, as it is
expected from every vector instruction that completes successfully.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20240314175704.478276-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit d3646e31ce6d1e02e46e6eabdbc2e637c0cbece7)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>