Abstract the liveness checking in PHIElimination::SplitPHIEdges() to support
both LiveVariables and LiveIntervals.
Fix the unused but nearly correct method SlotIndexes::insertMBBInMaps() and add
support for updating SlotIndexes to MachineBasicBlock::SplitCriticalEdge(). This
calls renumberIndexes() every time; it should be improved to only renumber
locally.