RegisterScavenging: Followup to r305625

RegisterScavenging: Followup to r305625

This does some improvements/cleanup to the recently introduced
scavengeRegisterBackwards() functionality:

- Rewrite findSurvivorBackwards algorithm to use the existing
  LiveRegUnit::accumulateBackward() code. This also avoids the Available
  and Candidates bitset and just need 1 LiveRegUnit instance
  (= 1 bitset).
- Pick registers in allocation order instead of register number order.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305817 91177308-0d34-0410-b5e6-96231b3b80d8