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

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

Right, this is not a NFC, since we were previously not taking into account all the visited MemoryAccess in `isWriteAtEndOfFunction`. From a quick glance at the diffs of a few samples, this should be OK.

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