Fix use of as_const in std_expr

[tautschnig]

Using as_const() on op() is too late, as op() itself would already
perform non-const operations. This is a fix-up to c267421.

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• 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).
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• My PR is restricted to a single feature or bugfix.
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[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 46f33b2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/104494333
Status will be re-evaluated on next push.
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).

• the compatibility was already broken by an earlier merge.

[allredj]

⚠️
This PR failed Diffblue compatibility checks (cbmc commit: 6be13f7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/104497036
Status will be re-evaluated on next push.
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).

• the compatibility was already broken by an earlier merge.