RISC-V: Fix avl-type operand index error for ZVBC

This patch fix the rtl-checking error for crypto vector. The root
cause is the avl-type index of zvbc ins is error,it should be operand[8]
not operand[5].
gcc/ChangeLog:

	* config/riscv/vector.md: Modify avl_type operand index of zvbc ins.