rv64v: fix decode for mask and whole register load/store instructions

[Ziyue-Zhang]

• previous implementation may recognize the wrong bitpat
• the nf field of vlm/vsm must be zero, the vm field must be 1
• the nf field of vlr/vsr must be 1/2/4/8, the vm field must be 1
• remove useless paramaters for vector whole register and index load/store functions