Revert "Convert liveness tracking to work on a sub-register level instead of just register units."

This reverts commit r197253.

This was a great change, but Juergen should be the commit author.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197262 91177308-0d34-0410-b5e6-96231b3b80d8