target-m68k: Inline shifts

Also manage word and byte operands and fix the computation of
overflow in the case of M68000 arithmetic shifts.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1478699171-10637-4-git-send-email-rth@twiddle.net>
