JIT: Use binary search in FlowGraphNaturalLoops::GetLoopByHeader

[amanasifkhalid]

Follow-up to #108086. Since loops are stored in reverse post-order in FlowGraphNaturalLoops::m_loops, loop headers' post-order numbers are stored in descending order. Thus, we can use binary search, and since we plan to use loop-aware RPOs in more places, we should probably take advantage of this.

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[amanasifkhalid]

Diffs show some small TP regressions, presumably from most methods having too few loops to overcome the overhead of binary search. I think this cost is acceptable if it helps us avoid excessive runtime for extreme cases, though for cases with fewer than N loops, should we fall back to linear search, or is that overkill? @jakobbotsch

[jakobbotsch]

Diffs show some small TP regressions, presumably from most methods having too few loops to overcome the overhead of binary search. I think this cost is acceptable if it helps us avoid excessive runtime for extreme cases, though for cases with fewer than N loops, should we fall back to linear search, or is that overkill? @jakobbotsch

I think it's fine to take the small regression.