Respect mixed return and raise cases in return-type analysis

[charliermarsh]

Summary

Given:

from somewhere import get_cfg

def lookup_cfg(cfg_description):
    cfg = get_cfg(cfg_description)
    if cfg is not None:
        return cfg
    raise AttributeError(f"No cfg found matching {cfg_description}")

We were analyzing the method from last-to-first statement. So we saw the raise, then assumed the method always raised. In reality, though, it might return. This PR improves the branch analysis to respect these mixed cases.

Closes #9269.
Closes #9304.

[github-actions]

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