[llvm] Reland "[LICM] Sink unused l-invariant loads in preheader #157559" (PR #170204)

[Nikita Popov via llvm-commits]

https://github.com/nikic requested changes to this pull request.

Maybe I'm missing something, but the MSSA updating looks wrong to me.

I haven't looked at the register pressure heuristic yet.

https://github.com/llvm/llvm-project/pull/170204