Allow SymbolicArray to represent arrays of non-header/header union types

[kfcripps]

For the following parser:

parser p(packet_in packet, out metadata_t metadata) {
    state start { transition s0; }
    state s0 {
        transition select(packet.lookahead<bit<8>>()) {
            1 : s1;
            default : accept;
        }
    }
    state s1 {
        packet.extract(metadata.h);
        metadata.array[1].f1 = metadata.h.array[0].f1;
        transition accept;
    }
}

Without these changes, the output of --top4 MidEndLast looks like this:

parser p(packet_in packet, out metadata_t metadata) {
    @name("p.tmp_0") bit<8> tmp_0;
    state stateOutOfBound {
        verify(false, error.StackOutOfBounds);
        transition reject;
    }
    state s1 {
        packet.extract<h_t>(metadata.h);
        transition stateOutOfBound;
    }
    state start {
        tmp_0 = packet.lookahead<bit<8>>();
        transition select(tmp_0) {
            8w1: s1;
            default: accept;
        }
    }
}

and with the changes:

parser p(packet_in packet, out metadata_t metadata) {
    @name("p.tmp_0") bit<8> tmp_0;
    state s1 {
        packet.extract<h_t>(metadata.h);
        metadata.array[1].f1 = metadata.h.array[0].f1;
        transition accept;
    }
    state start {
        tmp_0 = packet.lookahead<bit<8>>();
        transition select(tmp_0) {
            8w1: s1;
            default: accept;
        }
    }
}

Without the changes, metadata.array[1].f1 = metadata.h.array[0].f1; assignment incorrectly gets removed.