Revert "RegScavenging: Add scavengeRegisterBackwards()"

The ppc64 multistage bot fails on this.

This reverts commit r279124.

Also Revert "CodeGen: Add/Factor out LiveRegUnits class; NFCI" because it depends on the previous change
This reverts commit r279171.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279199 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed