Fix compilation statistics collection in inliner.

Stats from callee graph builder were not merged into main
stats and stats for callee graph optimizations were counted
even when the callee graph was eventually rejected.

Allocate the callee graph statistics on the arena.
Measured compilation of a big app using heaptrack:
  bytes allocated in total (ignoring deallocations): 3.77GB -> 3.37GB
  calls to allocation functions: 10650510 -> 8203129

Test: testrunner.py --host
Test: Stats change in the expected direction for an app.
Bug: 34053922
Change-Id: I605280d262b86af14b847acf3bb6dc077b749cc0