target-s390x: fix CC computation for EX instruction

Commit 7a6c7067f optimized CC computation by only saving cc_op before
calling helpers as they either don't touch the CC or generate a new
static value. This however doesn't work for the EX instruction as the
helper changes or not the CC value depending on the actual executed
instruction (e.g. MVC vs CLC).

This patches force a CC computation before calling the helper. This
fixes random memory corruption occuring in guests.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
[agraf: remove set_cc_static in op_ex as suggested by rth]
Signed-off-by: Alexander Graf <agraf@suse.de>
