RegAllocGreedy has been the default for six months now.
Deleting RegAllocLinearScan makes it possible to also delete
VirtRegRewriter and clean up the spiller code.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144475 91177308-0d34-0410-b5e6-96231b3b80d8