Incorrect Temp variable elimination

The Temp variable elimination may incorrectly remove some instructions:

```
external(cell1[0]) x;
external(cell1[1]) y;

inline void foo(ref a, in out b)
    a++;
    b++;
end;

foo(ref x, out y);
```

compiles to

```
read *tmp4 cell1 0
op add *tmp0 *tmp4 1
write *tmp0 cell1 0
op add :foo:b :foo:b 1
write :foo:b cell1 1
```
