target/riscv: Fix check for vector load/store instructions when EEW=64

target/riscv: Fix check for vector load/store instructions when EEW=64

The V extension supports all vector load and store instructions except
the V extension does not support EEW=64 for index values when XLEN=32.
(Section 18.3)

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20230215020539.4788-13-liweiwei@iscas.ac.cn>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>