Add additional optimizations to SimplifySelectCases midend pass

[kfcripps]

Reduces the following parser code:

    state state_1 {
        transition select(packet.lookahead<bit<16>>()) {
                0x21 : state_3;
                0x21 : state_2;
                default : accept;
        }
    }

to:

    state state_1 {
        transition select(packet.lookahead<bit<16>>()) {
                0x21 : state_3;
                default : accept;
        }
    }

and reduces the following parser code:

    state state_1 {
        transition select(packet.lookahead<bit<16>>()) {
                0x21 : state_3;
                0x22 : state_2;
                default : state_3;
        }
    }

to:

    state state_1 {
        transition select(packet.lookahead<bit<16>>()) {
                0x22 : state_2;
                default : state_3;
        }
    }

[ChrisDodd]

This seems to be checking for an actual constant, not a compile-time constant (compile-time constants also include directionless parameters that will be bound to a constant in an instantiation), so seems misnamed. perhaps just isConstant?