ARM: `opnd_create_base_disp` and `opnd_create_base_disp_aarch64` produce different results

The difference is `pre_index` value which is `true` from `opnd_create_base_disp` and `false` from `opnd_create_base_disp_aarch64`.

As a result DynamoRIO fails to encode the following code snippet:
```
opnd_t dst = opnd_create_reg(DR_REG_XZR);
opnd_t ptr = opnd_create_reg(DR_REG_X9);
opnd_t mem = opnd_create_base_disp(DR_REG_X9, DR_REG_NULL, 0, 0, OPSZ_PTR);
instr_t *i = instr_create_2dst_3src(dc, OP_ldr, dst, ptr, mem, ptr, OPND_CREATE_INT(-8));
instr_encode_to_copy(drcontext, i, buf, 0);
```

An error is like ` %xzr %x9'`

It's ok if I use `opnd_create_base_disp_aarch64` for both pre-indexing and post-indexing modes.