Use hybrid_binary(...) to avoid code duplication [blocks: #3867]

[tautschnig]

The implementation in as_mode.cpp was effectively doing the same, just using
different variable names.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 951d809).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/98025316