[llvm] [DeadStoreElimination] Refactor out `pushMemUses`, drop dead check (NFC) (PR #98530)

[Nikita Popov via llvm-commits]

nikic wrote:

Yes, it's not NFC, but I think the new code is more correct (previously I think we had some duplicates in the worklist, which made it more likely to hit the cutoff point).

https://github.com/llvm/llvm-project/pull/98530