The dominator tree gained an optimization to use block numbers instead
of a DenseMap to store blocks. Given that machine basic blocks already
have numbers, expose these via appropriate GraphTraits. For debugging,
block number epochs are added to MachineFunction -- this greatly helps
in finding uses of block numbers after RenumberBlocks().
In a few cases where dominator trees are preserved across renumberings,
the dominator tree is updated to use the new numbers.
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