Dead Code Elimination

[Walter Bright]

```d
int foo() {
     int x = 3;
     if (x == 2)
         return 4;
     return 0;
}
```


dmd -c -vasm -m32 dead.d:
```
_D4dead3fooFZi:
0000:   B8 03 00 00 00           mov       EAX,3
0005:   83 F8 02                 cmp       EAX,2
0008:   75 02                    jne       Lc
000a:   40                       inc       EAX
000b:   C3                       ret
000c:   31 C0                    xor       EAX,EAX
000e:   C3                       ret

```

dmd -c -vasm -O -m32 dead.d:

```
_D4dead3fooFZi:
0000:   31 C0                    xor       EAX,EAX
0002:   C3                       ret
```