[PATCH] Initial instrumentation based PGO implementation

[Justin Bogner]

Justin Bogner <mail at justinbogner.com> writes:
> Since we're basically just improving comments at this point, and since
> this not being in has been blocking other people trying to use and
> build on top of these changes, I'm going to go ahead and commit
> now. I'll continue to incorporate further feedback in follow on
> commits.

r198638, r198639, and r198640.