FEATURE: add support break always on level change

[cjmyers]

The always sensitivity list should allow break to happen when a =
continuous relation changes value to enable examples like the one below =
to be encoded.

Begin forwarded message:
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From: V.Dubikhin [email protected]
Subject: Re: Link
Date: August 31, 2016 at 7:24:54 PM GMT+1
To: Chris Myers [email protected]
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Property:
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property ADC_Freq{
real adc_in;
real adc_out;
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bool high;
bool low;
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always {
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if ((adc_in >=3D 0)) {
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if (~high) {
high =3D true;
wait(adc_out >=3D 0);
low =3D false;
}
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assert(adc_out>=3D0, 5);
assert(adc_out<0, 10);
} else {
if (~low) {
low =3D true;
wait(adc_out >=3D 0);
high =3D false;
}
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assert(adc_out>=3D0, 10);
assert(adc_out<0, 5);
}
}