target/riscv: Fix vslidedown with rvv_ta_all_1s

target/riscv: Fix vslidedown with rvv_ta_all_1s

vslidedown always zeroes elements past vl, where it should use the
tail policy.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250414213006.3509058-1-antonb@tenstorrent.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable@nongnu.org
(cherry picked from commit 2669b696e243b64f8ea1a6468dcee255de99f08d)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>