Consider the following scenario:

```
int a = 2;            /* constant assingment */
int b = a;            /* assignment via constant representation */
int c = a + b;
int d = c + 8;        /* mixed assigment */
return a + b + c + d; /* chained assignment */
```

The current code generator emits the assembly below, even though the
variables a, b, c, d are all constant representations.

```
mov     r0, #2
mov     r1, r0
mov     r0, r1
add     r3, r1, r0
mov     r2, sysprog21#8
add     r2, r3, r2
add     r4, r1, r0
add     r0, r4, r3
add     r1, r0, r2
mov     r0, r1
```

The new constant optimizer will evaluate the constant representations,
trim the unused variables, and emit `mov r0, sysprog21#20` only.