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.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[amanasifkhalid]

/azp run runtime-coreclr outerloop, Fuzzlyn, Antigen

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[amanasifkhalid]

cc @dotnet/jit-contrib, @AndyAyersMS PTAL. No diffs, and nominal TP impact (though TP diffs seem to be having some infra issues). Thanks!