Addition is the only operator that can change its return type during Lowering. Consider:

function f() {
    var i = 17;
    var j = 18;
    var k;
    for (;;) {
k = i + j;
i = {};
    }
    
}
Here, the addition i + j will be computed as ((double)i) + ((double)j) as that is the type used for integer additions (i = 17,j = 18), but when i is reassigned object, the addition remains, even though it should have been converted to a ScriptRuntime.ADD.

function runScript() {
 function f() {
  [|2|];
  var i = ((object)17) [i (type=O) (slot=4)];
  [|3|];
  var j = 18 [j (type=I) (slot=5)];
  [|4|];
  var k [k (type=O) (slot=6)];
  [|5|];
   {
   for (; ; ) {
    [|6|];
    DISCARD(k = (object)((double)i) + ((double)j));
    [|7|];
    DISCARD(i = {});
   } [GOTO] [TERMINAL]
  } [GOTO] [TERMINAL]
 }

We need to keep track of additions based on type changing source variables. I believe that the add operator is unique here as it is the only "either object or numeric return value" operator that exists.