10.4.4.26/27 Definite assignment rules for && and || should not apply to custom operators

See https://github.com/dotnet/roslyn/issues/4509 for background.

The clever rules around `||` and `&&` which allow for some "extra" definite assignments should _not_ be applied when using a custom `&` operator (and `false` operator) because otherwise they can lead to some weird cases where we'd allow reading from unassigned variables.