C++: Improve handling of re-use expressions

[jketema]

This will slightly brake IR generation for databases created with CodeQL 2.17.1. There are two changes:

• Some type are different in the IR. This seems inconsequential.
• The IR becomes inconsistent when a null pointer is being deleted. This should hopefully not occur at all in real world code.

[MathiasVP]

LGTM!

[jketema]

@MathiasVP note I added some additional text to the top-level comment.

[MathiasVP]

@MathiasVP note I added some additional text to the top-level comment.

Thanks for adding that piece of context. I think that's fine. AFAIK, we don't have any queries that:

• Use the IR, and
• Look for null pointer dereferences like that.

So, as you said yourself, I don't think this will have any impact on real-world DBs.