[SystemZ] Use LivePhysRegs helper class in SystemZShortenInst.cpp.

Don't use home brewed liveness tracking code for phys regs, since
this class does the job.

Reviewed by Ulrich Weigand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@250829 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed