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

[Florian Hahn via llvm-commits]

fhahn wrote:

> > @antoniofrighetto might be good to add a test case from [dtcxzyw/llvm-opt-benchmark#930](https://github.com/dtcxzyw/llvm-opt-benchmark/issues/930) if possible
> 
> Added in [10d720b](https://github.com/llvm/llvm-project/commit/10d720b5b4383c9900caaed7edf38ae31e8b9ac4), thanks!

Many thanks!

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