[SystemZ] Bugfix and improve the handling of CC values.
It was recently discovered that the handling of CC values was actually broken
since overflow was not properly handled ('nsw' flag not checked for).
Add and sub instructions now have a new target specific instruction flag
named SystemZII::CCIfNoSignedWrap. It means that the CC result can be used
instead of a compare with 0, but only if the instruction has the 'nsw' flag
This patch also adds the improvements of conversion to logical instructions
and the analyzing of add with immediates, to be able to eliminate more
Review: Ulrich Weigand