[bug] SimplifyDefUse removes used assignments

[mihaibudiu]

SimplifyDefUse needs to perform inter-procedural analysis for tables and actions:

control Ing() {
    bool b;
    bit<32> a;

    action cond() {
        if (b)  // <<< use of b
           a = 5;
        else
           a = 10;
    }

    apply {
        b = true;  // <<< incorrectly removed
        cond();
    }
}