RegScavenging: Add scavengeRegisterBackwards()

RegScavenging: Add scavengeRegisterBackwards()

Re-apply r276044/r279124. Trying to reproduce or disprove the ppc64
problems reported in the stage2 build last time, which I cannot
reproduce right now.

This is a variant of scavengeRegister() that works for
enterBasicBlockEnd()/backward(). The benefit of the backward mode is
that it is not affected by incomplete kill flags.

This patch also changes
PrologEpilogInserter::doScavengeFrameVirtualRegs() to use the register
scavenger in backwards mode.

Differential Revision: http://reviews.llvm.org/D21885

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305516 91177308-0d34-0410-b5e6-96231b3b80d8