[llvm] [MemorySSA] Make getPreviousDef iterative to avoid stack overflow (PR #205159)

[Alina Sbirlea via llvm-commits]

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

As a general (future) best practice, please push multiple commits so the history of the changes is preserved and reviewers can see the diff from the latest revision.
The multiple commits will get squashed into a single one when the PR is merged to main.

https://github.com/llvm/llvm-project/pull/205159