Revert "RegScavenging: Add scavengeRegisterBackwards()"

Revert because of reports of some PPC input starting to spill when it
was predicted that it wouldn't and no spillslot was reserved.

This reverts commit r305516.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305566 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed