[llvm] [MemorySSA] Handle MemoryDef optimized away during cloning (PR #117883)

[Florian Hahn via llvm-commits]

https://github.com/fhahn approved this pull request.

LGTM, making `getNewDefiningAccessForClone` safe seems preferable to working around the issue locally IMO.

https://github.com/llvm/llvm-project/pull/117883