Evaluate/Reevaluate the execution semantics of `CompletionRecord`

[nekevss]

As mentioned in PR #2618, we have previously been evaluating Return statements as normal completions and Yield statements a non-normal completion (ReturnType::Yield and now CompletionRecord::Return). This should be a different case with Yield statements being normal completions (CompletionRecord::Normal) and return statements being return completions (CompletionRecord::Return).

Completing the above change appears to require more evaluation/reevaluation of Generator executions and/or CompletionRecord semantics in the Vm as when the change was initially attempted, a number of test262 tests failed.