Make LoopAnalyzer compute loop bodies and depth only.
Move the logic for finding out loop headers and pre-headers to LoopInfo, which provides a visitor to iterate over the loops and easy access to the information.
This does not change the core algorithm.

BUG=None
R=jpp@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/2149803005 .