[llvm] [LoopInterchange] Consider eligible inner subnests (PR #214920)

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MattPD wrote:

@madhur13490 Thanks, FWIW, I'm still waiting to see what shape PR #199511 lands in before deciding which changes are still needed here (so no need review this version yet: I can ping you after I rebase after 199511 and after the changes; given the overlap I expect I'll be removing most of the candidate routing and then re-working the patch on top of that).

https://github.com/llvm/llvm-project/pull/214920