sim: bfin: handle odd shift values with shift insns

The shift magnitude is a 5-bit signed value.  When it is between 0 and
15, then we do the requested shift, but when it is outside of that, we
have to do the opposite.

That means we flip between lshift and ashiftrt to match the hardware.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>