LiveRegUnits: Add accumulateBackward() function

Re-Commit r292543 with a fix for the situation when the chain end is
MBB.end().

This function can be used to accumulate the set of all read and modified
register in a sequence of instructions.

Use this code in AArch64A57FPLoadBalancing::scavengeRegister() to prove
the concept.

- The AArch64A57LoadBalancing code is using a backwards analysis now
  which is irrespective of kill flags. This is the main motivation for
  this change.

Differential Revision: http://reviews.llvm.org/D22082

llvm-svn: 292705