Preparation for collapsing the two byte swaps adjust_endianness and
handle_bswap into the former.

Call memory_region_dispatch_{read|write} with endianness encoded into
the "MemOp op" operand.

This patch does not change any behaviour as
memory_region_dispatch_{read|write} is yet to handle the endianness.

Once it does handle endianness, callers with byte swaps can collapse
them into adjust_endianness.
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NTony Nguyen <tony.nguyen@bt.com>
Message-Id: <8066ab3eb037c0388dfadfe53c5118429dd1de3a.1566466906.git.tony.nguyen@bt.com>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>