bpo-33772: Fix couple of dead code paths

[ghost]

https://bugs.python.org/issue33772

[methane]

In ast.c and compile.c, break before next case seems worth.
It notes that "this case clause doesn't fallthrough".

Others seems good to me.

[ghost]

In ast.c and compile.c they always return before the break

[methane]

In ast.c and compile.c they always return before the break

I know. But the case clause is not so simple. It takes several seconds to read code for knowing it.

[ghost]

I hope I m not asking too much :-) but any opinion on this older PR ?

#6286

[csabella]

Closing and re-opening to retrigger CI checks.

[csabella]

@dcarlier-afilias, thank you for the pull request and, @methane, thank you for the code review and approval.