Add optimization removing unused temporaries

[bbannier]

We currently generate temporaries which depending on user code might be dead. We should look into an optimization to remove them. E.g., for the following unit

public type X = unit {
    : uint8[] &until=True;
};

we generate code for the &until condition

local bool __stop = False;
{
    local uint<8> __dd = __elem;
    __stop = __stop || True;
}

Here we create a copy of the current element in $$ so user code has access to it. This might not be needed and depending on the element type be an expensive operation we could avoid.