Use more specific return type instead of generic codet/exprt

[tautschnig]

If functions can only ever return a single kind of codet/exprt, there is no need
to use the generic codet/exprt and std::move.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 65efd76).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90305411