[CodeGen] Track trip counts per-scop for performance measurement.

- Add a counter that is incremented once on exit from a scop.

- Test cases got split into two: one to test the cycles, and another one
to test trip counts.

- Sample output:
```name=sample-output.txt
scop function, entry block name, exit block name, total time, trip count
warmup, %entry.split, %polly.merge_new_and_old, 5180, 1
f, %entry.split, %polly.merge_new_and_old, 409944, 500
g, %entry.split, %polly.merge_new_and_old, 1226, 1
```

Differential Revision: https://reviews.llvm.org/D33822

llvm-svn: 304543