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

[Alina Sbirlea via llvm-commits]

https://github.com/alinas commented:

The algorithm LGTM.
I added a couple of small notes inline, for overall this is great. Thank you!

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