[llvm] [SCEV] Add poison/undef canonicalization for SCEVAddExpr, SCEVAddRecExpr. (PR #170741)

[Philip Reames via llvm-commits]

https://github.com/preames commented:

This makes sense to me, but I agree we probably want to update most of the tests to not use undef, and then add specific tests for the new folds.  Maybe give others a chance to comment before you do the rework on the tests though?  

https://github.com/llvm/llvm-project/pull/170741