Optimize divide to use int divide when floating point result isn't needed

Motivated by perf issues in this microbenchmark brought up in #3828. We should be able to figure out that result is only used as an int, and therefore use int division.

````js
const buffer = new Uint16Array(5000000);

start = new Date();

function bar() {
for (let i = 0, len = buffer.length; i < len; i++) {
let idiv = 1;
let size = 1;

if (i) {
idiv = i;
size = i << 16;
}

const n = i * 8 / 6;

buffer[i] = n;
}
}
for (let i = 0; i < 100; ++i) {
bar()
}
console.log(new Date() - start)
````