JIT: Track visited blocks in LSRA with post-order number-based BitVec

[amanasifkhalid]

Since LSRA's block order computes a RPO traversal, we conveniently have post-order numbers on reachable blocks that can be used as keys in visited sets, and computing post-order numbers for unreached blocks is trivial. By using bbPostorderNum as the key instead of bbNum, we're one step closer to removing bbNum and BasicBlockEpoch.

cc @dotnet/jit-contrib

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[amanasifkhalid]

@jakobbotsch PTAL (once you're no longer OOF). No diffs, and slight TP regressions from the extra book-keeping needed to get this to work. I assume the removal of subsequent bbNum dependencies will easily pay for the TP cost. Thanks!