[llvm] [SCEV] Form AddRecs for PHIs involving values from nested loops. (PR #215658)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Marking this as a draft because it needs more work.  From https://github.com/llvm/llvm-project/issues/211229 , apparently using getSCEVAtScope() like this depends on the order SCEV is computed.  Maybe createAddRecFromPHI ends up recursively calling itself, and we end up bailing out.  Maybe we can mess with the getOperandsToCreate() to visit the loop-invariant operand first.

https://github.com/llvm/llvm-project/pull/215658