Mark failed blocks in root scope after all the tentative load checking is done.
Undebugged implementation of the commit phase of tentative load / phi / disabled-return checks.
Partially debug tentative load commit phase, and plug two minor design holes:

* Being used by an unannotated, unexpanded call now makes pointers escape
* Return values for unexpanded calls are tested if there's a dubious load in scope, not if there's a yield call, because a yield in specialised code could cause a should-check load in the call.