Use is_not_zero in simplifier

[tautschnig]

Avoid duplicating its implementation to reduce code size and improve
maintainability.

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[allredj]

Passed Diffblue compatibility checks (cbmc commit: b98e818).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87949806

[martin-cs]

is_not_zero does do the right thing with floats, right?

[tautschnig]

is_not_zero does do the right thing with floats, right?

It does have dedicated handling for that case, which amounts to the same that the code removed here was doing. But then I need to to defer back to you to determine whether that is the right thing...

[martin-cs]

ISO 9899:2011, 6.3.1.2/1: "When any scalar value is converted to _Bool, the result is 0 if the value compares equal to 0; otherwise, the result is 1."