[mypyc] Refactor method IR generation (#9846)
Add new helpers for generating methods that reduces the required
boilerplate. Switch several existing use cases to use the new helpers.
Summary of the new helpers:
1. Call `builder.enter_method(...)` to begin generating IR for a method.
2. Call `builder.add_argument(..)` for each non-self argument.
3. Generate method body normally.
4. Call `builder.leave_method()`.
Previously `enter()` and `leave()` had to be used, along with several
additional steps, such as defining `self` explicitly. The new approach
is much easier to use and less error-prone.
Since not all uses of the old interface have been removed, there is
still some duplication. It would be great to eventually always use the
higher-level interface to generate methods and functions.
Work on mypyc/mypyc#781.
