[llvm] [DSE] Refactor DSE (PR #100956)

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

with just this patch, it's still one MemoryLocation per MemoryDef. can we keep that assumption for now in this PR? and then make the change in the initializes DSE patch

https://github.com/llvm/llvm-project/pull/100956