Refactor: {Lvalue,Rvalue,Operand}::ty only need the locals' types, not the full &Mir
I am writing code that needs to call these `ty` methods while mutating MIR -- which is impossible with the current API.
Even with the refactoring the situation is not great: I am cloning the `local_decls` and then passing the clone to the `ty` methods. I have to clone because `Mir::basic_blocks_mut` borrows the entire `Mir` including the `local_decls`. But even that is better than not being able to get these types at all...
Cc @nikomatsakis