Use constructors to construct objects

[tautschnig]

This is some drive-by cleanup resulting from reviewing #3067. Instead of
incrementally constructing objects do RAII.

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[smowton]

One simple improvement given the code-function-callt-with-dummy-first-argument pattern occurs a few times (though perhaps this would introduce an ambiguous first constructor?) Also as noted in the other PR, I'd prefer if move-to-operands -> add was not pessimised.

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 11d34d3).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86479380
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)
• the author is not in the list of contributors (e.g. first-time contributors).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 0ee10a4).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86484504

[allredj]

Passed Diffblue compatibility checks (cbmc commit: dbc5439).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86678835