[CVP] Adjust iteration order to reduce the amount of work required

CVP doesn't care about the order of blocks visited, but by using a pre-order traversal over the graph we can a) not visit unreachable blocks and b) optimize as we go so that analysis of later blocks produce slightly more precise results.

I noticed this via inspection and don't have a concrete example which points to the issue.  

llvm-svn: 290760