C++: Also handle reverse flow on instructions in dataflow #14384
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#14376 fixed a bug where we incorrectly shared representation between certain dataflow nodes.
This fix revealed another bug: the "reverse flow" mechanism (which we have for detecting that something like
myMap["abc"] = source();is a write that modifiesmyMap) didn't account for the case where theOutNodethat represents the result ofmyMap["abc"]was an instruction instead of an operand.This bug was hidden until #14376 was merged because these operand and instruction nodes accidentally shared representation.
OutNodes are operands most of the time, but they're instructions when the result of theCallInstructionhas multiple uses. One of the cases where this happens after the frontend upgrade is when an assignment (which in this case was a call to an overloaded = operator) happens as an argument to a function call such asfoo(m11["abc"] = source());.