Make regression test pass when sizeof(size_t) == sizeof(int)

[tautschnig]

The type cast previously tested for in the regression test will only be
introduced when the type of an array index is not the same as int. Thus the test
spuriously failed on 32-bit x86 platforms.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: 6620cdb).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/86427120