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.

llvm-svn: 305817