simplify_function_application: do not assume that a symbol always has a value

[tautschnig]

The possible arrays we might see include dynamic objects, which are symbols, but
need not have an initial value. While at it, actually implement a proper prefix
check rather than limiting this to the same length. And then do take into
account the offset, which was previously missing as well.

This is exercised by several startsWith regression tests, but the initial problem was only triggered when enabling further constant propagation.

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[smowton]

Could you add 0 conditions remaining or whatever it is we say when symex has solved the problem to those startsWith tests?

[tautschnig]

Could you add 0 conditions remaining or whatever it is we say when symex has solved the problem to those startsWith tests?

All of them seem to be crafted such that there is at least one failing assertion, which does require the use of SAT solver. I have now extended the test that @JohnDumbell had added for this initial version of this simplifier pass.

[allredj]

🚫
This PR failed Diffblue compatibility checks (cbmc commit: 00f9de3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99643033
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
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The incompatibility may have been introduced by an earlier PR. In that case merging this
PR should be avoided unless it fixes the current incompatibility.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 79c76a7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99646614