[mem2reg] llvm.structured.alloca support

We need handling of the `llvm.structured.alloca` intrinsic in mem2reg. I think that we can probably do this in the existing pass without too much disruption rather than needing a separate pass like we did with LSROA, but we could pivot in the other direction depending on the implementation details.

There is some further context in https://discourse.llvm.org/t/rfc-adding-logical-structured-alloca/90027 and https://github.com/llvm/llvm-project/pull/192058.