Cleanup: Refactor `rustc_typeck/check/op.rs`

[Centril]

rustc_typeck/check/op.rs has some seriously over-complicated methods, e.g. fn check_overloaded_binop, that obscure the happy path and do way too much. The file and especially the aforementioned method should be refactored.

[ljedrz]

I'd like to take a stab at this if that's ok.