support for updating SlotIndexes to MachineBasicBlock::SplitCriticalEdge(). This
calls renumberIndexes() every time; it should be improved to only renumber
locally.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174851 91177308-0d34-0410-b5e6-96231b3b80d8