Avoid exhausting stack space in dominator compression
Doesn't add a test case -- I ended up running into this while playing with the generated example from #43578, which we could do with a run-make test (to avoid checking a large code snippet into tree), but I suspect we don't want to wait for it to compile (locally it takes ~14s -- not terrible, but doesn't seem worth it to me). In practice stack space exhaustion is difficult to test for, too, since if we set the bound too low a different call structure above us (e.g., a nearer ensure_sufficient_stack call) would let the test pass even with the old impl, most likely.
Locally it seems like this manages to perform approximately equivalently to the recursion, but will run perf to confirm.
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