ARM: `drutil_insert_get_mem_addr` no longer works correctly on AArch64

Created by: petrochenkov

https://github.com/DynamoRIO/dynamorio/pull/5283 introduced a change to decoding of load instructions, as a result of that change load index registers can be decoded as w registers (as opposed to x registers as previously).

Not all places in DR are prepared to encounter a w register in a load index context. For example, this logic in drutil_insert_get_mem_addr_arm

        reg_id_t index = opnd_get_index(memref);

        // ...

        reg_id_t stolen = dr_get_stolen_reg();

        // ...

        } else if (index == stolen) {
            index = replace_stolen_reg(drcontext, bb, where, memref, dst, scratch,
                                       scratch_used);
        }

now doesn't recognize w index registers as stolen, because dr_get_stolen_reg() typically returns an x register.

This results in instrumentation being silently incorrect and drutil_insert_get_mem_addr_arm producing incorrect addresses.

Some other places besides drutil_insert_get_mem_addr_arm may also be affected by this change.