[Support] Store dominator tree nodes in a vector (#101705)

Use basic block numbers to store dominator tree nodes in a vector. This
avoids frequent map lookups. Use block number epochs to validate t

[Support] Store dominator tree nodes in a vector (#101705)

Use basic block numbers to store dominator tree nodes in a vector. This
avoids frequent map lookups. Use block number epochs to validate that no
renumbering occured since the tree was created; after a renumbering, the
dominator tree can be updated with updateBlockNumbers().

Block numbers, block number epoch, and max block number are fetched via
newly added GraphTraits methods. Graphs which do not implement
getNumber() for blocks will use a DenseMap for an ad-hoc numbering.

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