Incorrect codegen in arithmetic expressions in REPLs

[xTrayambak]

Sample

let x = 3
console.log(x) // Log: 3
let y = x + 5
console.log(y) // Log: null
console.log(x) // Log: null

The value at x is magically reduced to atoms (no pun intended). This is because the IR generator incorrectly generates a MOVE opcode when it should really emit COPY instead. We could implement the current behaviour as a heuristical optimization later.