fix accesses to exprt::opX() in goto-programs/

[kroening]

This improves type safety.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 2b18b82).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/123388518

[smowton]

Did #5025; @peterschrammel would you mind looking at this one?

[martin-cs]

There are still some op0() and op1() calls which could probably be removed.