Do not use arbitrarily bounded integer types [blocks: #2310]

[tautschnig]

We generate expressions, there is no need to restrict values to machine-specific
bounded integer types.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: c32a775).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90944917