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

[Alina Sbirlea via llvm-commits]

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alinas wrote:

Nit: `SmallVector<TrackingVH<MemoryAccess>, 4>`, in most code BBs have less than 8 predecessors.

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