The best resources on understanding Polly are probably various LLVM developer conference talks (it’s come up at nearly every one since at least 2010 or so).

But just like not needing to worry too much about how GCC does this unless you’re a compiler engineer, I’d say just either let it try or not.

Edit: Here's the 2011 talk (http://llvm.org/devmtg/2011-11/Grosser_PollyOptimizations.pd...) that I think is a good yet old intro. Tobias's more recent work (say http://www.grosser.es/bibliography/doerfert2017optimistic.ht...) is a good "what's possible now-ish". But again, these are both compiler-researcher papers.