JIT: Use postorder numbers as keys into ordinals in 3-opt layout

[amanasifkhalid]

Now that most fgRenumberBlocks calls are gone, bbNum-indexed arrays are likely to be quite sparse by the time we get to the backend. Using postorder numbers in lieu of bbNum during block layout is more efficient; to do this, we have to keep references to the initial RPO traversal around throughout the phase, though these data structures might be useful for influencing future decisions 3-opt may make as we expand its capabilities.

This change was motivated by my efforts to support dumping CFG data that can be consumed by external TSP optimizers -- the one I'm considering requires nodes to be numbered with contiguous indices, which postorder numbers are good for.

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[amanasifkhalid]

/azp run runtime-coreclr outerloop, Fuzzlyn, Antigen

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