Refactor a CallGraphPropertyAnalysis helper [NFC] (#2441)

This moves code out of Asyncify into a general helper class. The class
automates scanning the functions for a property, then propagating it to
functions that call them. In Asyncify, the property is "may call something
that leads to sleep", and we propagate backwards to callers, to find
all those that may sleep.

This will be useful in a future exceptions-optimizing pass I want to write,
where the property will be "may throw". We will then be able to remove
exceptions overhead in cases that definitely do not throw.

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