Fixed incorrect division results

tests/programs/operator-assign.q

@@ -0,0 +1,30 @@
+main() {
+	number(10)
+	register(10)
+}
+
+number(x int) {
+	y := x
+	x -= 1
+	assert x < y
+	x += 1
+	assert x == y
+	x *= 2
+	assert x > y
+	x /= 2
+	assert x == y
+}
+
+register(x int) {
+	y := x
+	num := 1
+	x -= num
+	assert x < y
+	x += num
+	assert x == y
+	num = 2
+	x *= num
+	assert x > y
+	x /= num
+	assert x == y
+}