Fix some problems with the updating of SlotIndexes after adding a new MBB. In
particular, holes were being left between two blocks after splitting an edge.
Update SlotIndexes after updateTerminator() possibly removes instructions. I am
really trying to avoid piping SlotIndexes through to RemoveBranch() and friends.
Add support for updating LiveIntervals to MachineBasicBlock::SplitCriticalEdge().
This is currently a bit hairier than it needs to be, since depending on where the
split block resides the end ListEntry of the split block may be the end ListEntry
of the original block or a new entry. Some changes to the SlotIndexes updating
should make it possible to eliminate the two cases here.

This also isn't as optimized as it could be. In the future Liveinterval should
probably get a flag that indicates whether the LiveInterval is within a single
basic block. We could ignore all such intervals when splitting an edge.