Revert "Remove fno-tree-vrp compile flag."

This reverts commit e8d6e3fe3b85f88fe9506257b0cbbf5341a3a840.
Also added a comment pointing to the recent discussion.
See http://www.freelists.org/post/haiku-development/hrev45320-Yet-another-nonobvious-effect-of-ftreevrp-optimization