[llvm] [SCCIterator] Union MST node by rank correctly (PR #86389)

[David Blaikie via llvm-commits]

dwblaikie wrote:

> > Any chance of a test case?
> 
> It's a bit hard because this change only improves the efficiency of union find, instead of correctness.

Ah, "correctly" in the title sounded like there was a correctness issue (perhaps one that harms performance more broadly - but perhaps the correctness aspect could be reached via unit test or the like?).

https://github.com/llvm/llvm-project/pull/86389