JIT: Add loop-aware RPO, and use as LSRA's block sequence

[amanasifkhalid]

Part of #107749, and follow-up to #107927. When computing a RPO of the flow graph, ensuring that the entirety of a loop body is visited before any of the loop's successors has the benefit of keeping the loop body compact in the traversal. This is certainly ideal when computing an initial block layout, and may be preferable for register allocation, too. Thus, this change formalizes loop-aware RPO creation as part of the flowgraph API surface, and uses it for LSRA's block sequence.

I plan to reuse the RPO computed during LSRA in fgDoReversePostOrderLayout once #107634 is in. To do this, I had to add a new phase check flag to disable checking basic block pre/postorder numbers, since the loop-aware RPO (or just a profile-aware RPO) won't match up with the expected DFS in our debug checks -- it seems simplest to just disable these checks altogether once we reach the backend.

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[amanasifkhalid]

I don't think we have any dependencies on bbPreorderNum or bbPostorderNum in the backend, but if we want to use loop-aware RPOs elsewhere in the JIT, I can work on making these members consistent.