Fix the updating of LiveIntervals after splitting a critical edge. PHI operand
live ranges should always be extended, and the only successor that should be
considered for extension of other ranges is the target of the split edge.
Add blocks to the LiveIntervalAnalysis RegMaskBlocks array when splitting
a critical edge.
Renumber SlotIndexes locally when a new block is inserted.
Add a hidden option to PHIElimination to split all critical edges. This is
particularly useful for catching issues with architectures that have exotic
terminators like MIPS.