target/riscv: rvv: Source vector registers cannot overlap mask register

target/riscv: rvv: Source vector registers cannot overlap mask register

Add the relevant ISA paragraphs explaining why source (and destination)
registers cannot overlap the mask register.

Signed-off-by: Anton Blanchard <antonb@tenstorrent.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Max Chou <max.chou@sifive.com>
Signed-off-by: Max Chou <max.chou@sifive.com>
Message-ID: <20250408103938.3623486-2-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable@nongnu.org
(cherry picked from commit 3e8d1e4a628bb234c0b5d1ccd510900047181dbd)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>