target-arm: fix bug in translation of REVSH

The translation of REVSH shifted the low byte 8 steps left before performing
an 8-bit sign extend, causing this part of the expression to alwas be 0.

Reported-by: Johan Bengtsson <teofrastius@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>