Abstract the liveness checking in PHIElimination::SplitPHIEdges() to support
both LiveVariables and LiveIntervals.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174850 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174851 91177308-0d34-0410-b5e6-96231b3b80d8