C++: Prevent cached stages from being re-evaluated
C++: Prevent cached stages from being re-evaluated

Before this change, evaluating `cpp/constant-comparison` followed by
`cpp/signed-overflow-check` would result in re-evaluation of almost all
the cached stages they share: CFG, basic blocks, SSA, and range
analysis. The same effect could be seen on `cpp/bad-strncpy-size`, which
also uses the GVN library.

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