Fix use of binary irep in smt2_conv

[kroening]

The conversion has still assumed that bitvector contants are represented in
binary. This now uses the integer2bvrep interface.

Fixes #4066.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: dea63c9).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99743896

[tautschnig]

Looking at, for example, cbmc/Float24 the code handling floating-point constants still appears to be very much broken, but at least it isn't obvious anymore...

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: a73ac9d).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99764225