[SystemZ] Address review comments for r233689

Change lowerCTPOP to:
- Gracefully handle a known-zero input value
- Simplify computation of significant bit size

Thanks to Jay Foad for the review!



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233736 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed