Based on comments from Chris, switch to using CFG::getNumBlockIDs()

Based on comments from Chris, switch to using CFG::getNumBlockIDs()
rather than a computed std::distance(). At some point I had convinced
myself that these two were different; but as far as I can tell on
re-exampination they aren't, and the number of block IDs is actually
just a count of the blocks in the CFG.

While this removes the primary motivation for guarding all of this with
CollectStats, I have a patch coming up that will almost certainly make
it important again.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134552 91177308-0d34-0410-b5e6-96231b3b80d8