Stopper analysis

This patch teaches souper to tell us what is stopping it from building larger LHSs. Example output below. I believe this is useful instrumentation to have siting around.

```
   2 souper      - Stopped due to alloca
1726 souper      - Stopped due to call
  11 souper      - Stopped due to extract value
4884 souper      - Stopped due to load
1117 souper      - Stopped due to loop
  16 souper      - Stopped due to select on a non-integer
   4 souper      - Stopped due to unsupported constant
1141 souper      - Stopped for another reason
```