JIT: Don't move blocks during return merging

[amanasifkhalid]

While removing the old block layout code, I noticed return merging has a step for moving return blocks up to their ideal predecessors to make up for deficiencies in block reordering that no longer exist. Removing this step adds jumps to newly added return blocks in MinOpts, and incurs a few diffs from fgUpdateFlowGraph churn in FullOpts; both seem acceptable.

[Copilot]

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[amanasifkhalid]

Since we limit the number of epilogues to 4 at the moment, the TP cost of emitting additional branches due to the loss of fallthrough in MinOpts exceeds the TP gain from not moving the return blocks up to their successors. It's not worth pursuing this now, but it's worth reconsidering if we ever increase the number of epilogues allowed.