Example:
```
nand. r1, r2, r3
```
Assuming that none of these registers will be used again:
- Before #14278, all of them would be flushed on both sides of the branch.
- After #14278, r2 and r3 would have been flushed earlier, but r1 won't.
- With this, r1 is flushed in ComputeRC.
Martino Fontana
e68a464efd