[llvm] [DeadStoreElimination] Optimize tautological assignments (PR #75744)

[Nikita Popov via llvm-commits]

nikic wrote:

Compile-time looks good: http://llvm-compile-time-tracker.com/compare.php?from=f0db35b93f31ea5d6ff9bd4791fb6755b5a5bb9b&to=14257f5968d784e5f21b768afbaf8b53d211fd22&stat=instructions:u

https://github.com/llvm/llvm-project/pull/75744