Value-set filtering: gracefully handle pointer arithmetic

[tautschnig]

Pointer arithmetic can yield expressions that are deemed constant for
the purpose of constant propagation (such as &x + 1), even though they
aren't actually constant_exprt.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e739fbb).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/112912971