Move definition of dr_pred_type_t to opnd.h for AArch64

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`).

Search the repository for the definition and uses of dr_pred_type_t, then inspect opnd.h and the current AArch64 operand declarations. Move the definition into opnd.h and update affected function signatures or references so the enum type is used instead of int; confirm the project still builds and its relevant tests pass.

Written by the indexing model from the issue text.