fix for arraylength

[kroening]

The type of the array for arraylength doesn't matter -- the previous
implementation uses the first letter of the statement, which only happens to
be a valid type character ('a').

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 409d0fc).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/114775588