Move definition of dr_pred_type_t to opnd.h for AArch64

Created by: yury-khrustalev

The definition of dr_pred_type_t for AArch64 should reside in opnd.h as the elements of this enum describe one of the types of operands of AArch64 instructions. This will allows to define functions which work with this kind of operand properly (i.e. using dr_pred_type_t instead of int).