[SystemZ] Bugfix: check CC reg liveness in SystemZShortenInst.

The following instruction shortening transformations would introduce a
definition of the CC reg, so therefore liveness of CC reg must be checked:

WFADB -> ADBR
WFSDB -> SDBR

Also add the CC reg implicit def operand to the MI in case of change of opcode.

Reviewed by Ulrich Weigand.

llvm-svn: 249665