RegisterScavenger: Introduce backward() mode.

This adds two pieces:
- RegisterScavenger:::enterBasicBlockEnd() which behaves similar to
  enterBasicBlock() but starts tracking at the end of the basic block.
- A RegisterScavenger::backward() method. It is subtly different
  from the existing unprocess() method which only considers uses with
  the kill flag set: If a value is dead at the end of a basic block with
  a last use inside the basic block, unprocess() will fail to mark it as
  live. However we cannot change/fix this behaviour because unprocess()
  needs to perform the exact reverse operation of forward().

Differential Revision: http://reviews.llvm.org/D21873

llvm-svn: 276043