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

[Florian Hahn via llvm-commits]

fhahn wrote:

(some of the tests probably need updating to add variants without undef/poison, which the canonicalization is desirable)

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