Track the number of cached/recomputed values using the LLVM statistics macro

https://llvm.org/docs/ProgrammersManual.html#id38

```
#include "llvm/ADT/Statistic.h"

STATISTIC(NumCachedValues, "Number of values cached");
STATISTIC(NumRecomputedValues, "Number of values recomputed");
```

Other stats to track:
- GPU gradient accumulation
- Non-atomic load/store
- Parallel Reduction
- Atomic increment
- Allocation Merging
- Heap-to-stack (and register)